## 6.4.3 Constraints on an Instance

Objects and Variables may change their Attribute values after being created. Special rules apply for some Attributes as defined in 6.2.6.

Additional References may be added to the Nodes, and References may be deleted as long as the ModellingRules defined on the InstanceDeclarations of the TypeDefinitionNode are still fulfilled.

For Variables and Objects the HasTypeDefinition Reference shall always point to the same TypeDefinitionNode as the InstanceDeclaration or a subtype of it.

If two InstanceDeclarations of the fully-inherited InstanceDeclarationHierarchy have been connected directly with several References, all those References shall connect the same Nodes. An example is given in Figure 23. The instances A1 and A2 are allowed since B1 references the same Node with both References, whereas A3 is not allowed since two different Nodes are referenced. Note that this restriction only applies for directly connected Nodes. For example, A2 references a C1 directly and a different C1 via B1.

Figure 23 – Example for several References between InstanceDeclarations