If no ModellingRule exists then the Node is neither considered for instantiation of a type nor for subtyping.
If a Node referenced by a TypeDefinitionNode does not reference a ModellingRule it indicates that this Node only belongs to the TypeDefinitionNode and not to the instances. For example, an ObjectType Node may contain a Property that describes scenarios where the type could be used. This Property would not be considered when creating instances of the type. This is also true for subtyping, that is, subtypes of the type definition would not inherit the referenced Node.