At the provides side the interface associated via the outer role is only allowed to provide the same or reduced interface like the interface associated via the inner role. For those constellations all calls on services of the outer interface can be delegated to defined services of the inner interface.
At the requires side interoperability conditions are vice versa. The outer interface can define services that actually are not required by the inner interface, but there are no service calls of the inner interface that can not be delegated to the outer interface.