[Currently a release group can be a tribute to an artist. But there are also release groups that are tributes to specific albums, for example:

Flaming Lips - With a Little Help From My Fwends (tribute to Sgt Pepper)

Noam Pikelny - Plays Kenny Baker Plays Bill Monroe (tribute to Kenny Baker Plays Bill Monroe)

Various Artists - Tribute to the Anthology of American Folk Music by Harry Smith and also The Harry Smith Project: Anthology of American Folk Music Revisited (both tributes to the Anthology of American Folk Music)

Vitamin String Quartet - More Bricks: The String Quartet Tribute to Pink Floyd’s The Wall