When adding a relationship between release groups the PartOfASet-releationship is available.

If this is intended to be used only between releases it should be hidden for release groups.
If this relationship can also be used between release groups to define Series the description of the relationship should be changed for release groups. This way it could replace http://wiki.musicbrainz.org/Series .

http://test.musicbrainz.org/edit/relationship/create?type1=release_group&entity0=8f630cc5-162e-4ec7-b6a8-6aa6ed017eb1&entity1=7f89a360-407a-49f3-84b0-ad6680de7dbc&type0=release_group