I recently asked a similar question about instrumentals and while at first I thought it was a bit odd that “instrumental of” was a recording-work relationship and not a recording-recording relationship I have since gotten used to it.
So what about acapellas? Why is there no recording-work relationship for those? It would make sense to me if it was the same relationship type as for instrumentals, but I didn’t find any relationship for it anywhere. Btw is it just me or do searches in the documentation never yield any results?