diff --git a/SDT/schema4.0/docs/SDT_UML.uxf b/SDT/schema4.0/docs/SDT_UML.uxf
index cc0b7e3a989eff7d6d92da1ba152e3df890fca36..0900afc744e8704888d7a1fbdeeb8615fa9ef2f3 100644
--- a/SDT/schema4.0/docs/SDT_UML.uxf
+++ b/SDT/schema4.0/docs/SDT_UML.uxf
@@ -5,8 +5,8 @@
   <element>
     <id>UMLNote</id>
     <coordinates>
-      <x>1152</x>
-      <y>1424</y>
+      <x>1112</x>
+      <y>1440</y>
       <w>232</w>
       <h>208</h>
     </coordinates>
@@ -36,8 +36,8 @@ group=1</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1256</x>
-      <y>1504</y>
+      <x>1216</x>
+      <y>1520</y>
       <w>88</w>
       <h>32</h>
     </coordinates>
@@ -51,8 +51,8 @@ group=1</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1256</x>
-      <y>1536</y>
+      <x>1216</x>
+      <y>1552</y>
       <w>88</w>
       <h>24</h>
     </coordinates>
@@ -64,8 +64,8 @@ group=1</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>1264</x>
-      <y>1056</y>
+      <x>1224</x>
+      <y>1072</y>
       <w>120</w>
       <h>176</h>
     </coordinates>
@@ -88,8 +88,8 @@ void</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>768</x>
-      <y>1120</y>
+      <x>728</x>
+      <y>1136</y>
       <w>112</w>
       <h>64</h>
     </coordinates>
@@ -101,8 +101,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>768</x>
-      <y>1056</y>
+      <x>728</x>
+      <y>1072</y>
       <w>112</w>
       <h>64</h>
     </coordinates>
@@ -114,8 +114,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>384</x>
-      <y>1056</y>
+      <x>344</x>
+      <y>1072</y>
       <w>152</w>
       <h>120</h>
     </coordinates>
@@ -134,8 +134,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>528</x>
-      <y>1016</y>
+      <x>488</x>
+      <y>1032</y>
       <w>544</w>
       <h>96</h>
     </coordinates>
@@ -147,8 +147,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>528</x>
-      <y>1016</y>
+      <x>488</x>
+      <y>1032</y>
       <w>544</w>
       <h>144</h>
     </coordinates>
@@ -160,8 +160,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>864</x>
-      <y>1152</y>
+      <x>824</x>
+      <y>1168</y>
       <w>152</w>
       <h>40</h>
     </coordinates>
@@ -174,8 +174,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1008</x>
-      <y>1176</y>
+      <x>968</x>
+      <y>1192</y>
       <w>272</w>
       <h>32</h>
     </coordinates>
@@ -187,8 +187,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>864</x>
-      <y>1248</y>
+      <x>824</x>
+      <y>1264</y>
       <w>152</w>
       <h>88</h>
     </coordinates>
@@ -205,8 +205,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>528</x>
-      <y>1136</y>
+      <x>488</x>
+      <y>1152</y>
       <w>352</w>
       <h>144</h>
     </coordinates>
@@ -218,8 +218,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1008</x>
-      <y>1224</y>
+      <x>968</x>
+      <y>1240</y>
       <w>336</w>
       <h>112</h>
     </coordinates>
@@ -231,8 +231,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>864</x>
-      <y>1056</y>
+      <x>824</x>
+      <y>1072</y>
       <w>152</w>
       <h>40</h>
     </coordinates>
@@ -245,8 +245,8 @@ fg=blue</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>864</x>
-      <y>1104</y>
+      <x>824</x>
+      <y>1120</y>
       <w>152</w>
       <h>40</h>
     </coordinates>
@@ -259,8 +259,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>768</x>
-      <y>1096</y>
+      <x>728</x>
+      <y>1112</y>
       <w>112</w>
       <h>40</h>
     </coordinates>
@@ -273,8 +273,8 @@ fontsize=12
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>384</x>
-      <y>944</y>
+      <x>344</x>
+      <y>960</y>
       <w>1000</w>
       <h>32</h>
     </coordinates>
@@ -288,8 +288,8 @@ lw=0.1</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>96</x>
-      <y>72</y>
+      <x>56</x>
+      <y>88</y>
       <w>1288</w>
       <h>32</h>
     </coordinates>
@@ -303,15 +303,14 @@ lw=0.1</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>664</x>
-      <y>176</y>
+      <x>624</x>
+      <y>192</y>
       <w>192</w>
-      <h>168</h>
+      <h>152</h>
     </coordinates>
     <panel_attributes>ModuleClass
 --
 *@ name : Name*
-/@ optional : boolean = false/
 /@ semanticURI : uri/
 /@ minOccurs : text = "1"/
 /@ maxOccurs : text = "1"/
@@ -328,8 +327,8 @@ fg=blue
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>944</x>
-      <y>176</y>
+      <x>904</x>
+      <y>192</y>
       <w>176</w>
       <h>104</h>
     </coordinates>
@@ -347,8 +346,8 @@ fg=blue</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>1216</x>
-      <y>248</y>
+      <x>1176</x>
+      <y>264</y>
       <w>168</w>
       <h>104</h>
     </coordinates>
@@ -366,8 +365,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1112</x>
-      <y>248</y>
+      <x>1072</x>
+      <y>264</y>
       <w>120</w>
       <h>32</h>
     </coordinates>
@@ -379,8 +378,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>112</x>
-      <y>176</y>
+      <x>72</x>
+      <y>192</y>
       <w>184</w>
       <h>128</h>
     </coordinates>
@@ -400,8 +399,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>176</y>
+      <x>504</x>
+      <y>192</y>
       <w>136</w>
       <h>64</h>
     </coordinates>
@@ -413,8 +412,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>376</x>
-      <y>424</y>
+      <x>336</x>
+      <y>440</y>
       <w>176</w>
       <h>120</h>
     </coordinates>
@@ -433,8 +432,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>272</y>
+      <x>504</x>
+      <y>288</y>
       <w>56</w>
       <h>72</h>
     </coordinates>
@@ -446,21 +445,21 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
+      <x>504</x>
       <y>352</y>
       <w>136</w>
-      <h>128</h>
+      <h>144</h>
     </coordinates>
     <panel_attributes>lt=&lt;.
 m1= 0..n
 fontsize=12</panel_attributes>
-    <additional_attributes>150.0;10.0;80.0;10.0;80.0;140.0;10.0;140.0</additional_attributes>
+    <additional_attributes>150.0;10.0;80.0;10.0;80.0;160.0;10.0;160.0</additional_attributes>
   </element>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>1216</x>
-      <y>472</y>
+      <x>1176</x>
+      <y>488</y>
       <w>168</w>
       <h>40</h>
     </coordinates>
@@ -472,8 +471,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>848</x>
-      <y>176</y>
+      <x>808</x>
+      <y>192</y>
       <w>112</w>
       <h>56</h>
     </coordinates>
@@ -485,8 +484,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1168</x>
-      <y>472</y>
+      <x>1128</x>
+      <y>488</y>
       <w>64</w>
       <h>32</h>
     </coordinates>
@@ -498,8 +497,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>848</x>
-      <y>224</y>
+      <x>808</x>
+      <y>240</y>
       <w>112</w>
       <h>96</h>
     </coordinates>
@@ -511,8 +510,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>944</x>
-      <y>288</y>
+      <x>904</x>
+      <y>304</y>
       <w>176</w>
       <h>144</h>
     </coordinates>
@@ -535,8 +534,8 @@ fg=blue
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>944</x>
-      <y>440</y>
+      <x>904</x>
+      <y>456</y>
       <w>176</w>
       <h>88</h>
     </coordinates>
@@ -553,8 +552,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>848</x>
-      <y>304</y>
+      <x>808</x>
+      <y>320</y>
       <w>112</w>
       <h>168</h>
     </coordinates>
@@ -566,8 +565,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>176</y>
+      <x>504</x>
+      <y>192</y>
       <w>136</w>
       <h>336</h>
     </coordinates>
@@ -579,8 +578,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>376</x>
-      <y>312</y>
+      <x>336</x>
+      <y>328</y>
       <w>176</w>
       <h>104</h>
     </coordinates>
@@ -598,8 +597,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>392</y>
+      <x>504</x>
+      <y>408</y>
       <w>64</w>
       <h>64</h>
     </coordinates>
@@ -611,8 +610,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>176</y>
+      <x>504</x>
+      <y>192</y>
       <w>136</w>
       <h>224</h>
     </coordinates>
@@ -623,18 +622,18 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
+      <x>504</x>
       <y>352</y>
       <w>136</w>
-      <h>32</h>
+      <h>48</h>
     </coordinates>
     <panel_attributes>lt=&lt;.</panel_attributes>
-    <additional_attributes>150.0;10.0;80.0;10.0;80.0;20.0;10.0;20.0</additional_attributes>
+    <additional_attributes>150.0;10.0;80.0;10.0;80.0;40.0;10.0;40.0</additional_attributes>
   </element>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>664</x>
+      <x>624</x>
       <y>352</y>
       <w>192</w>
       <h>104</h>
@@ -654,8 +653,8 @@ transparency=80</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1112</x>
-      <y>176</y>
+      <x>1072</x>
+      <y>192</y>
       <w>56</w>
       <h>320</h>
     </coordinates>
@@ -667,21 +666,21 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>848</x>
-      <y>304</y>
+      <x>808</x>
+      <y>320</y>
       <w>56</w>
-      <h>80</h>
+      <h>64</h>
     </coordinates>
     <panel_attributes>lt=&lt;.
 m1=0..n
 fontsize=12</panel_attributes>
-    <additional_attributes>10.0;70.0;40.0;70.0;40.0;10.0;10.0;10.0</additional_attributes>
+    <additional_attributes>10.0;50.0;40.0;50.0;40.0;10.0;10.0;10.0</additional_attributes>
   </element>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>624</x>
-      <y>1064</y>
+      <x>584</x>
+      <y>1080</y>
       <w>152</w>
       <h>88</h>
     </coordinates>
@@ -699,8 +698,8 @@ Enum : EnumType
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>528</x>
-      <y>1072</y>
+      <x>488</x>
+      <y>1088</y>
       <w>112</w>
       <h>64</h>
     </coordinates>
@@ -712,20 +711,20 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>240</y>
+      <x>504</x>
+      <y>256</y>
       <w>136</w>
-      <h>136</h>
+      <h>120</h>
     </coordinates>
     <panel_attributes>lt=&lt;.
 fontsize=12</panel_attributes>
-    <additional_attributes>150.0;150.0;80.0;150.0;80.0;10.0;10.0;10.0</additional_attributes>
+    <additional_attributes>150.0;130.0;80.0;130.0;80.0;10.0;10.0;10.0</additional_attributes>
   </element>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>288</x>
-      <y>144</y>
+      <x>248</x>
+      <y>160</y>
       <w>392</w>
       <h>88</h>
     </coordinates>
@@ -736,8 +735,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>544</x>
-      <y>256</y>
+      <x>504</x>
+      <y>272</y>
       <w>64</w>
       <h>192</h>
     </coordinates>
@@ -749,8 +748,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>864</x>
-      <y>1200</y>
+      <x>824</x>
+      <y>1216</y>
       <w>152</w>
       <h>40</h>
     </coordinates>
@@ -763,8 +762,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>768</x>
-      <y>1128</y>
+      <x>728</x>
+      <y>1144</y>
       <w>112</w>
       <h>104</h>
     </coordinates>
@@ -776,8 +775,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>1064</x>
-      <y>1216</y>
+      <x>1024</x>
+      <y>1232</y>
       <w>168</w>
       <h>88</h>
     </coordinates>
@@ -794,8 +793,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1008</x>
-      <y>1216</y>
+      <x>968</x>
+      <y>1232</y>
       <w>72</w>
       <h>32</h>
     </coordinates>
@@ -807,8 +806,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1224</x>
-      <y>1224</y>
+      <x>1184</x>
+      <y>1240</y>
       <w>88</w>
       <h>64</h>
     </coordinates>
@@ -820,8 +819,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>432</x>
-      <y>696</y>
+      <x>392</x>
+      <y>712</y>
       <w>176</w>
       <h>64</h>
     </coordinates>
@@ -837,8 +836,8 @@ fg=blue
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>928</x>
-      <y>696</y>
+      <x>888</x>
+      <y>712</y>
       <w>192</w>
       <h>56</h>
     </coordinates>
@@ -852,8 +851,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>856</x>
-      <y>696</y>
+      <x>816</x>
+      <y>712</y>
       <w>88</w>
       <h>56</h>
     </coordinates>
@@ -866,8 +865,8 @@ fontsize=12
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>384</x>
-      <y>696</y>
+      <x>344</x>
+      <y>712</y>
       <w>64</w>
       <h>32</h>
     </coordinates>
@@ -879,8 +878,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>1208</x>
-      <y>696</y>
+      <x>1168</x>
+      <y>712</y>
       <w>176</w>
       <h>128</h>
     </coordinates>
@@ -900,8 +899,8 @@ subdevice
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1112</x>
-      <y>696</y>
+      <x>1072</x>
+      <y>712</y>
       <w>112</w>
       <h>56</h>
     </coordinates>
@@ -913,8 +912,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>928</x>
-      <y>760</y>
+      <x>888</x>
+      <y>776</y>
       <w>192</w>
       <h>56</h>
     </coordinates>
@@ -928,8 +927,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>856</x>
-      <y>744</y>
+      <x>816</x>
+      <y>760</y>
       <w>88</w>
       <h>48</h>
     </coordinates>
@@ -942,8 +941,8 @@ fontsize=12
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>1112</x>
-      <y>696</y>
+      <x>1072</x>
+      <y>712</y>
       <w>112</w>
       <h>120</h>
     </coordinates>
@@ -955,8 +954,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>688</x>
-      <y>696</y>
+      <x>648</x>
+      <y>712</y>
       <w>176</w>
       <h>64</h>
     </coordinates>
@@ -973,8 +972,8 @@ Includes : Include
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>600</x>
-      <y>704</y>
+      <x>560</x>
+      <y>720</y>
       <w>104</w>
       <h>56</h>
     </coordinates>
@@ -987,8 +986,8 @@ fontsize=12
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>376</x>
-      <y>176</y>
+      <x>336</x>
+      <y>192</y>
       <w>176</w>
       <h>128</h>
     </coordinates>
@@ -1009,8 +1008,8 @@ fg=blue</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>288</x>
-      <y>288</y>
+      <x>248</x>
+      <y>304</y>
       <w>104</w>
       <h>56</h>
     </coordinates>
@@ -1022,8 +1021,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>Relation</id>
     <coordinates>
-      <x>288</x>
-      <y>176</y>
+      <x>248</x>
+      <y>192</y>
       <w>104</w>
       <h>104</h>
     </coordinates>
@@ -1035,8 +1034,8 @@ fontsize=12</panel_attributes>
   <element>
     <id>UMLClass</id>
     <coordinates>
-      <x>384</x>
-      <y>608</y>
+      <x>344</x>
+      <y>624</y>
       <w>1000</w>
       <h>32</h>
     </coordinates>
diff --git a/SDT/schema4.0/docs/images/MC.Action.DataPoint.Event.png b/SDT/schema4.0/docs/images/MC.Action.DataPoint.Event.png
index 67ee56e2689fd24bfb444f281c7bcbc594121fe6..f30e52425bb3b97d5bca10d8bcea3fdaac447966 100644
Binary files a/SDT/schema4.0/docs/images/MC.Action.DataPoint.Event.png and b/SDT/schema4.0/docs/images/MC.Action.DataPoint.Event.png differ
diff --git a/SDT/schema4.0/docs/images/ModuleClass.png b/SDT/schema4.0/docs/images/ModuleClass.png
index 3c8fa19e10746abafa6c34400c6ab3a5744f8b50..0f59a32d5b2be297d3722e6c22994187e3b4668d 100644
Binary files a/SDT/schema4.0/docs/images/ModuleClass.png and b/SDT/schema4.0/docs/images/ModuleClass.png differ
diff --git a/SDT/schema4.0/docs/images/SDT_UML_Basic_Elements.png b/SDT/schema4.0/docs/images/SDT_UML_Basic_Elements.png
index 38fae6a92cde0f2c321e94963313494eb24dfd54..a740368c3ee9a099a579d3328106a3af3ee1585e 100644
Binary files a/SDT/schema4.0/docs/images/SDT_UML_Basic_Elements.png and b/SDT/schema4.0/docs/images/SDT_UML_Basic_Elements.png differ
diff --git a/SDT/schema4.0/docs/images/SDT_simplified.png b/SDT/schema4.0/docs/images/SDT_simplified.png
index f3ff40db0d91f40696e54e5a7e03d980b153214e..62e4f01c77ebfe6fdedab1af4e7eb293a51721a3 100644
Binary files a/SDT/schema4.0/docs/images/SDT_simplified.png and b/SDT/schema4.0/docs/images/SDT_simplified.png differ
diff --git a/SDT/schema4.0/etc/domain.rnc b/SDT/schema4.0/etc/domain.rnc
index 0b1a4d113ffbf52a2dc817fea14bc32e69961ffc..e09e19f5ad58bab8d7fa99ed67c1af6fcce27702 100644
--- a/SDT/schema4.0/etc/domain.rnc
+++ b/SDT/schema4.0/etc/domain.rnc
@@ -120,7 +120,6 @@ ModuleClassesDef =
   element ModuleClasses {
     element ModuleClass {
       attribute name { xsd:Name },
-      attribute optional { xsd:boolean }?,
       attribute semanticURI { xsd:anyURI }?,
       attribute minOccurs { text }?,
       attribute maxOccurs { text }?,
diff --git a/SDT/schema4.0/src/domain.rng b/SDT/schema4.0/src/domain.rng
index 267bc6d6841f8e1182c04f848f29416bfe4a8438..9ba0f59bfd9e6d6f99112f596392f88b080c2124 100644
--- a/SDT/schema4.0/src/domain.rng
+++ b/SDT/schema4.0/src/domain.rng
@@ -344,11 +344,6 @@
 					<attribute name="name">
 						<data type="Name"/>
 					</attribute>
-					<optional>
-						<attribute name="optional">
-							<data type="boolean"/>
-						</attribute>
-					</optional>
 					<optional>
 						<attribute name="semanticURI">
 							<data type="anyURI"/>
diff --git a/SDT/schema4.0/src/domain.xsd b/SDT/schema4.0/src/domain.xsd
index ff074e985bddda70c8f84e8e3fc2acf13ba00f9e..e3e8fad40dedfcb3747ec039d9118bd05a68b827 100644
--- a/SDT/schema4.0/src/domain.xsd
+++ b/SDT/schema4.0/src/domain.xsd
@@ -272,7 +272,6 @@
             <xs:element minOccurs="0" ref="Events"/>
           </xs:sequence>
           <xs:attribute name="name" use="required" type="xs:Name"/>
-          <xs:attribute name="optional" type="xs:boolean"/>
           <xs:attribute name="semanticURI" type="xs:anyURI"/>
           <xs:attribute name="minOccurs"/>
           <xs:attribute name="maxOccurs"/>
diff --git a/SDT/schema4.0/test/mseeb.xml b/SDT/schema4.0/test/mseeb.xml
index c729380aed91f7ffca0a21348eec5272f1a51a08..7ce9b37cdfacd6e38b1cec379328c3dbc426ddb7 100644
--- a/SDT/schema4.0/test/mseeb.xml
+++ b/SDT/schema4.0/test/mseeb.xml
@@ -154,7 +154,7 @@
 -->
 
             <ModuleClasses>
-                <ModuleClass name="rootPowerOnOff" optional="true">
+                <ModuleClass name="rootPowerOnOff" minOccurs="0" maxOccurs="1">
                     <Doc>
                         The proprietry interface for turning all switches of the extension block on and off. 
                         The action "BooleanState:get" returns "true" when any of the sub-devices is in the state "on", otherwise it returns "false".