New ModellingRules have to be defined if the predefined ModellingRules are not appropriate for the model exposed by the Server.

Depending on the model used by the underlying system the Server may need to define new ModellingRules, since the OPC UA Server may only pass the data to the underlying system and this system may use its own internal rules for instantiation, subtyping, etc.

Beside this, the predefined ModellingRules might not be sufficient to specify the required behaviour for instantiation and subtyping.