Occasionally a release will come out and an instrumental/karaoke version of the release will come out later. We currently have no way to link releases like this aside from annotations.
Example of a release-release relationship:
PRAHA has karaoke version PRAHA～BACK TRACKS～
PRAHA～BACK TRACKS～ is a karaoke version of PRAHA

Example of a release group-release group relationship:
PRAHA has karaoke version PRAHA～BACK TRACKS～
PRAHA～BACK TRACKS～ is a karaoke version of PRAHA

I should note that this relationship is not meant to be used for karaoke releases by artists other than the original (esp. anonymous releases by companies specializing in karaoke).