The subject is implemented by the app on the client side, while the OOUI Kit creates appropriate views and controllers on the server.
The subjects are implementations of the class interfaces created by the Model interface and implemented by the application.
XXX Specify the interface for updating observers of instances, i.e. notifying the kit of changes to the app data.
XXX How is/are the root reference(s) transfered to the kit?
Concrete relationships are specified similar to instances: The application creates instances for the Role/Relationship interfaces.