It is allowed to add subtypes to the existing ReferenceType hierarchy. It is not allowed to move types in that Hierarchy, e.g. making a hierarchical ReferenceType a non-hierarchical ReferenceType, as the filtering would behave differently. It is also not allowed to add new ReferenceTypes inside the Hierarchy. This would work for old OPC UA Clients, since they would just not use those unknown ReferenceTypes for filtering, but not for new OPC UA Clients connecting to old OPC UA Servers, since they would expect the ReferenceType to be available and use it for filtering information available in the OPC UA Server.