InstanceDeclarations can be added optionally on each place without the need to create subtypes. This does not only include InstanceDeclaration on an TypeDefinition directly, but also on an InstanceDeclaration of the TypeDefinition (without the need to change the type of the owning InstanceDeclaration). If InstanceDeclarations need to be added mandatory, either a subtype needs to be created with the mandatory InstanceDeclaration, or the InstanceDeclaration is only added optional and a new Conformance Unit requires the InstanceDeclaration to be present.