The descriptions of the co and re relationship attributes aren’t very useful (see the attached image). Of course their purpose is usually quite clear, but I remember there being a guideline (that I can’t find now of course) saying that co should only be used if it is explicitly mentioned on the release, and there may be more to re than meets the eye too.

My proposal for co:

This attribute indicates that artists cooperated on this relationship. Only use this if co is explicitly mentioned on the release, not for every relationship type that applies to more than one artist.

And for re:

This attribute indicates that a relationship was redone for this recording or release, for example a remaster.