Occasionally when an artist releases an album, a karaoke version of the album will be released (essentially the exact same album with no lead vocal tracks). In my opinion these albums should be separated out from their source albums (that's a discussion for another time), but currently there's no relationship to link the two release groups together.
Example: 246コネクション has a karaoke version ROUTE 246 CONNEXiON YOKO OGINOME “OFF VOCAL” SPECIAL