Grouping is formally only meant to be used for subtitles/subsets/sections of works/albums, but in practice Boroda is correct - it is very useful as a pseudo-custom tag for a multitude of things. If you use the grouping tag, consider using sub-headers (added way back in response to a request
(by me amongst many others) to be able to show sub-titles, disc numbers and the like in album and tracks view). This really unlocks the potential of it (see Bee-liever's guide
to help you on this matter).
Bold words in my posts are links unless expressly stated otherwise. Have fun with them!