diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 675069e4c..906266039 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -10,6 +10,8 @@
 
 === Improvements
 
+- https://github.com/eclipse-syson/syson/issues/829[#829] [metamodel] `OccurrenceUsag#portionKind` is now unsettable and its default value is `null`.
+
 === New features
 
 
diff --git a/backend/application/syson-application/src/test/resources/simple_project.json b/backend/application/syson-application/src/test/resources/simple_project.json
index 349356bc4..86286583c 100644
--- a/backend/application/syson-application/src/test/resources/simple_project.json
+++ b/backend/application/syson-application/src/test/resources/simple_project.json
@@ -224,7 +224,7 @@
         "isReference": "false",
         "isVariation": "false",
         "isIndividual": "false",
-        "portionKind": "snapshot",
+        "portionKind": null,
         "documentation": [],
         "ownedAnnotation": [],
         "ownedElement": [],
@@ -4621,4 +4621,4 @@
         "variantMembership": [],
         "lifeClass": null
     }
-]
\ No newline at end of file
+]
diff --git a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/OccurrenceUsage.java b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/OccurrenceUsage.java
index 56b11374c..3767bb049 100644
--- a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/OccurrenceUsage.java
+++ b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/OccurrenceUsage.java
@@ -84,9 +84,11 @@ public interface OccurrenceUsage extends Usage {
      *
      * @return the value of the '<em>Portion Kind</em>' attribute.
      * @see org.eclipse.syson.sysml.PortionKind
+     * @see #isSetPortionKind()
+     * @see #unsetPortionKind()
      * @see #setPortionKind(PortionKind)
      * @see org.eclipse.syson.sysml.SysmlPackage#getOccurrenceUsage_PortionKind()
-     * @model ordered="false"
+     * @model unsettable="true" ordered="false"
      * @generated
      */
     PortionKind getPortionKind();
@@ -98,9 +100,34 @@ public interface OccurrenceUsage extends Usage {
      * @param value
      *            the new value of the '<em>Portion Kind</em>' attribute.
      * @see org.eclipse.syson.sysml.PortionKind
+     * @see #isSetPortionKind()
+     * @see #unsetPortionKind()
      * @see #getPortionKind()
      * @generated
      */
     void setPortionKind(PortionKind value);
 
+    /**
+     * Unsets the value of the '{@link org.eclipse.syson.sysml.OccurrenceUsage#getPortionKind <em>Portion Kind</em>}'
+     * attribute. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @see #isSetPortionKind()
+     * @see #getPortionKind()
+     * @see #setPortionKind(PortionKind)
+     * @generated
+     */
+    void unsetPortionKind();
+
+    /**
+     * Returns whether the value of the '{@link org.eclipse.syson.sysml.OccurrenceUsage#getPortionKind <em>Portion
+     * Kind</em>}' attribute is set. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @return whether the value of the '<em>Portion Kind</em>' attribute is set.
+     * @see #unsetPortionKind()
+     * @see #getPortionKind()
+     * @see #setPortionKind(PortionKind)
+     * @generated
+     */
+    boolean isSetPortionKind();
+
 } // OccurrenceUsage
diff --git a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/ConnectionUsageImpl.java b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/ConnectionUsageImpl.java
index 5ee1758b6..3c05cbb89 100644
--- a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/ConnectionUsageImpl.java
+++ b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/ConnectionUsageImpl.java
@@ -81,10 +81,10 @@ public class ConnectionUsageImpl extends ConnectorAsUsageImpl implements Connect
      * <!-- end-user-doc -->
      *
      * @see #getPortionKind()
-     * @generated
+     * @generated NOT
      * @ordered
      */
-    protected static final PortionKind PORTION_KIND_EDEFAULT = PortionKind.SNAPSHOT;
+    protected static final PortionKind PORTION_KIND_EDEFAULT = null;
 
     /**
      * The cached value of the '{@link #getPortionKind() <em>Portion Kind</em>}' attribute. <!-- begin-user-doc --> <!--
@@ -96,6 +96,14 @@ public class ConnectionUsageImpl extends ConnectorAsUsageImpl implements Connect
      */
     protected PortionKind portionKind = PORTION_KIND_EDEFAULT;
 
+    /**
+     * This is true if the Portion Kind attribute has been set. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     * @ordered
+     */
+    protected boolean portionKindESet;
+
     /**
      * <!-- begin-user-doc --> <!-- end-user-doc -->
      *
@@ -191,8 +199,35 @@ public PortionKind getPortionKind() {
     public void setPortionKind(PortionKind newPortionKind) {
         PortionKind oldPortionKind = this.portionKind;
         this.portionKind = newPortionKind == null ? PORTION_KIND_EDEFAULT : newPortionKind;
+        boolean oldPortionKindESet = this.portionKindESet;
+        this.portionKindESet = true;
+        if (this.eNotificationRequired())
+            this.eNotify(new ENotificationImpl(this, Notification.SET, SysmlPackage.CONNECTION_USAGE__PORTION_KIND, oldPortionKind, this.portionKind, !oldPortionKindESet));
+    }
+
+    /**
+     * <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public void unsetPortionKind() {
+        PortionKind oldPortionKind = this.portionKind;
+        boolean oldPortionKindESet = this.portionKindESet;
+        this.portionKind = PORTION_KIND_EDEFAULT;
+        this.portionKindESet = false;
         if (this.eNotificationRequired())
-            this.eNotify(new ENotificationImpl(this, Notification.SET, SysmlPackage.CONNECTION_USAGE__PORTION_KIND, oldPortionKind, this.portionKind));
+            this.eNotify(new ENotificationImpl(this, Notification.UNSET, SysmlPackage.CONNECTION_USAGE__PORTION_KIND, oldPortionKind, PORTION_KIND_EDEFAULT, oldPortionKindESet));
+    }
+
+    /**
+     * <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public boolean isSetPortionKind() {
+        return this.portionKindESet;
     }
 
     /**
@@ -287,7 +322,7 @@ public void eUnset(int featureID) {
                 this.setIsIndividual(IS_INDIVIDUAL_EDEFAULT);
                 return;
             case SysmlPackage.CONNECTION_USAGE__PORTION_KIND:
-                this.setPortionKind(PORTION_KIND_EDEFAULT);
+                this.unsetPortionKind();
                 return;
         }
         super.eUnset(featureID);
@@ -304,7 +339,7 @@ public boolean eIsSet(int featureID) {
             case SysmlPackage.CONNECTION_USAGE__IS_INDIVIDUAL:
                 return this.isIndividual != IS_INDIVIDUAL_EDEFAULT;
             case SysmlPackage.CONNECTION_USAGE__PORTION_KIND:
-                return this.portionKind != PORTION_KIND_EDEFAULT;
+                return this.isSetPortionKind();
             case SysmlPackage.CONNECTION_USAGE__INDIVIDUAL_DEFINITION:
                 return this.basicGetIndividualDefinition() != null;
             case SysmlPackage.CONNECTION_USAGE__OCCURRENCE_DEFINITION:
@@ -413,7 +448,10 @@ public String toString() {
         result.append(" (isIndividual: ");
         result.append(this.isIndividual);
         result.append(", portionKind: ");
-        result.append(this.portionKind);
+        if (this.portionKindESet)
+            result.append(this.portionKind);
+        else
+            result.append("<unset>");
         result.append(')');
         return result.toString();
     }
diff --git a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/OccurrenceUsageImpl.java b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/OccurrenceUsageImpl.java
index 0d00fc62f..553b847e2 100644
--- a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/OccurrenceUsageImpl.java
+++ b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/OccurrenceUsageImpl.java
@@ -71,10 +71,10 @@ public class OccurrenceUsageImpl extends UsageImpl implements OccurrenceUsage {
      * <!-- end-user-doc -->
      *
      * @see #getPortionKind()
-     * @generated
+     * @generated NOT
      * @ordered
      */
-    protected static final PortionKind PORTION_KIND_EDEFAULT = PortionKind.SNAPSHOT;
+    protected static final PortionKind PORTION_KIND_EDEFAULT = null;
 
     /**
      * The cached value of the '{@link #getPortionKind() <em>Portion Kind</em>}' attribute. <!-- begin-user-doc --> <!--
@@ -86,6 +86,14 @@ public class OccurrenceUsageImpl extends UsageImpl implements OccurrenceUsage {
      */
     protected PortionKind portionKind = PORTION_KIND_EDEFAULT;
 
+    /**
+     * This is true if the Portion Kind attribute has been set. <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     * @ordered
+     */
+    protected boolean portionKindESet;
+
     /**
      * <!-- begin-user-doc --> <!-- end-user-doc -->
      *
@@ -181,8 +189,35 @@ public PortionKind getPortionKind() {
     public void setPortionKind(PortionKind newPortionKind) {
         PortionKind oldPortionKind = this.portionKind;
         this.portionKind = newPortionKind == null ? PORTION_KIND_EDEFAULT : newPortionKind;
+        boolean oldPortionKindESet = this.portionKindESet;
+        this.portionKindESet = true;
+        if (this.eNotificationRequired())
+            this.eNotify(new ENotificationImpl(this, Notification.SET, SysmlPackage.OCCURRENCE_USAGE__PORTION_KIND, oldPortionKind, this.portionKind, !oldPortionKindESet));
+    }
+
+    /**
+     * <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public void unsetPortionKind() {
+        PortionKind oldPortionKind = this.portionKind;
+        boolean oldPortionKindESet = this.portionKindESet;
+        this.portionKind = PORTION_KIND_EDEFAULT;
+        this.portionKindESet = false;
         if (this.eNotificationRequired())
-            this.eNotify(new ENotificationImpl(this, Notification.SET, SysmlPackage.OCCURRENCE_USAGE__PORTION_KIND, oldPortionKind, this.portionKind));
+            this.eNotify(new ENotificationImpl(this, Notification.UNSET, SysmlPackage.OCCURRENCE_USAGE__PORTION_KIND, oldPortionKind, PORTION_KIND_EDEFAULT, oldPortionKindESet));
+    }
+
+    /**
+     * <!-- begin-user-doc --> <!-- end-user-doc -->
+     *
+     * @generated
+     */
+    @Override
+    public boolean isSetPortionKind() {
+        return this.portionKindESet;
     }
 
     /**
@@ -238,7 +273,7 @@ public void eUnset(int featureID) {
                 this.setIsIndividual(IS_INDIVIDUAL_EDEFAULT);
                 return;
             case SysmlPackage.OCCURRENCE_USAGE__PORTION_KIND:
-                this.setPortionKind(PORTION_KIND_EDEFAULT);
+                this.unsetPortionKind();
                 return;
         }
         super.eUnset(featureID);
@@ -255,7 +290,7 @@ public boolean eIsSet(int featureID) {
             case SysmlPackage.OCCURRENCE_USAGE__IS_INDIVIDUAL:
                 return this.isIndividual != IS_INDIVIDUAL_EDEFAULT;
             case SysmlPackage.OCCURRENCE_USAGE__PORTION_KIND:
-                return this.portionKind != PORTION_KIND_EDEFAULT;
+                return this.isSetPortionKind();
             case SysmlPackage.OCCURRENCE_USAGE__INDIVIDUAL_DEFINITION:
                 return this.basicGetIndividualDefinition() != null;
             case SysmlPackage.OCCURRENCE_USAGE__OCCURRENCE_DEFINITION:
@@ -278,7 +313,10 @@ public String toString() {
         result.append(" (isIndividual: ");
         result.append(this.isIndividual);
         result.append(", portionKind: ");
-        result.append(this.portionKind);
+        if (this.portionKindESet)
+            result.append(this.portionKind);
+        else
+            result.append("<unset>");
         result.append(')');
         return result.toString();
     }
diff --git a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/SysmlPackageImpl.java b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/SysmlPackageImpl.java
index e998ff275..c3e9dd499 100644
--- a/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/SysmlPackageImpl.java
+++ b/backend/metamodel/syson-sysml-metamodel/src/main/java/org/eclipse/syson/sysml/impl/SysmlPackageImpl.java
@@ -9790,7 +9790,7 @@ public void initializePackageContents() {
         this.initEAttribute(this.getOccurrenceUsage_IsIndividual(), this.ecorePackage.getEBoolean(), "isIndividual", "false", 1, 1, OccurrenceUsage.class, !IS_TRANSIENT, !IS_VOLATILE, IS_CHANGEABLE,
                 !IS_UNSETTABLE,
                 !IS_ID, IS_UNIQUE, !IS_DERIVED, !IS_ORDERED);
-        this.initEAttribute(this.getOccurrenceUsage_PortionKind(), this.getPortionKind(), "portionKind", null, 0, 1, OccurrenceUsage.class, !IS_TRANSIENT, !IS_VOLATILE, IS_CHANGEABLE, !IS_UNSETTABLE,
+        this.initEAttribute(this.getOccurrenceUsage_PortionKind(), this.getPortionKind(), "portionKind", null, 0, 1, OccurrenceUsage.class, !IS_TRANSIENT, !IS_VOLATILE, IS_CHANGEABLE, IS_UNSETTABLE,
                 !IS_ID,
                 IS_UNIQUE, !IS_DERIVED, !IS_ORDERED);
         this.initEReference(this.getOccurrenceUsage_IndividualDefinition(), this.getOccurrenceDefinition(), null, "individualDefinition", null, 0, 1, OccurrenceUsage.class, IS_TRANSIENT, IS_VOLATILE,
diff --git a/backend/metamodel/syson-sysml-metamodel/src/main/resources/model/sysml.ecore b/backend/metamodel/syson-sysml-metamodel/src/main/resources/model/sysml.ecore
index 735055186..a6098c120 100644
--- a/backend/metamodel/syson-sysml-metamodel/src/main/resources/model/sysml.ecore
+++ b/backend/metamodel/syson-sysml-metamodel/src/main/resources/model/sysml.ecore
@@ -1226,7 +1226,7 @@
         lowerBound="1" eType="ecore:EDataType http://www.eclipse.org/emf/2002/Ecore#//EBoolean"
         defaultValueLiteral="false"/>
     <eStructuralFeatures xsi:type="ecore:EAttribute" name="portionKind" ordered="false"
-        eType="#//PortionKind"/>
+        eType="#//PortionKind" unsettable="true"/>
     <eStructuralFeatures xsi:type="ecore:EReference" name="individualDefinition" ordered="false"
         eType="#//OccurrenceDefinition" changeable="false" volatile="true" transient="true"
         derived="true">
diff --git a/doc/content/modules/user-manual/pages/release-notes/2025.1.0.adoc b/doc/content/modules/user-manual/pages/release-notes/2025.1.0.adoc
index c518ac798..12716bf1a 100644
--- a/doc/content/modules/user-manual/pages/release-notes/2025.1.0.adoc
+++ b/doc/content/modules/user-manual/pages/release-notes/2025.1.0.adoc
@@ -10,4 +10,6 @@
 
 == Improvements
 
+- `OccurrenceUsag#portionKind` is now unsettable and its default value is `null` in the SysMLv2 metamodel to conform to the specification.
+
 == New features