It is not allowed to add new mandatory InstanceDeclarations using the ModellingRules Mandatory and MandatoryPlaceholder. To add a mandatory InstanceDeclaration, either create a new Subtype having the mandatory InstanceDeclaration, or a new type independent of the previous one. As alternative, make the new InstanceDeclaration optional but define a Profile requiring the new InstanceDeclaration.

This rule avoids the incompatibility when an OPC UA Client aware of the new version connects to an OPC UA Server only supporting the old version, and the OPC UA Client expects a new mandatory feature not provided by the OPC UA Server.