An InstanceDeclarationmarked with the ModellingRule Optionalmeans that for each existing BrowsePathon the instance a similar Nodemay exist, but it is not defined whether a new Nodeis created or an existing Nodeis referenced.
In Figure 27an example using the ModellingRules Optionaland Mandatoryis shown. The example contains an ObjectTypeType_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 Nodeor two different Nodeswith the same BrowseName(similar Nodeto InstanceDeclarationF). Not considered in the example is if the instances have ModellingRulesor not. It is assumed that each F is similar to the InstanceDeclarationF, etc.
If there would be a NonHierarchical Referencebetween 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.