Something that uses more explicit language what a "relationship" means
between documents. It is a very vague term. I wouldn't have guessed
in a hundred years that it could pertain to copyright. Besides if one
looks up the currently defined registrations for REL, not any of the
eighteen or so listed types includes any mention of copyright.
In other words, I don't agree that the issue is clear. I don't see
any standardized vehicle for a publisher of a document to express
copyright except in human language or as an attached human language