Clarify the repositoryRelationship attribute on a workspace.

In the description of the Atom Service Document (REST) it says that the workspace will have a cmis attribute for repositoryRelationship. This is an enum, and its values are described in part 1 in the enums table (self, replica, peer, parent, child, archive). This seems to be described (vaguely) in part 1, page 19 where it says the relationship "MAY" be returned, and the name is opaque to cmis, and it gives some "suggested/example values". So if the values are opaque and suggested and optional, why does the REST Service document require it and constrain the value in an enum?