An InstanceDeclaration marked with the ModellingRule Optional fulfils exactly the semantic defined for the NamingRule Optional. That means that for each existing BrowsePath on the instance a similar Node may exist, but it is not defined whether a new Node is created or an existing Node is referenced.
In Figure 20 an example using the ModellingRules Optional and Mandatory is shown. The example contains an ObjectType Type_A and all valid combinations of instances named A1 to A13. Note that if the optional B is provided, the mandatory E has to be provided as well, otherwise not. F is referenced by C and D. On the instance, this can be the same Node or two different Nodes with the same BrowseName (similar Node to InstanceDeclaration F). Not considered in the example is if the instances have ModellingRules or not. It is assumed that each F is similar to the InstanceDeclaration F, etc.
If there would be a non-hierarchical Reference between E and F in the InstanceDeclarationHierarchy, it is not specified if it occurs in the instance hierarchy or not. In the case of A10, there could be a reference from E to one F but not to the other F, or to both or none of them.