I cannot simply double click the artist or an album (which sends the songs to the playlist) and listen to exactly which mastering I want.
If you click or double-click the heading row for the title of each mastering you can select or play, respectively, each one separately.
But this nesting idea can be applied to artist, genre, etc.
While I agree it would be a useful addition for the mastering scenario you have described, I can't think of too many other uses for it. Genres can be kinda hierarchical but they don't currently have a graphical view like artists and albums do, so it wouldn't apply here. I guess you could list individual performers separately from bands which could be useful for people in multiple projects.
But how would you set it up?
What would that look like?
A new MusicBee-specific tag for
Sub-Album?
Does that mean you need
Sub-Artist too?
Or some more generic multi-purpose nesting-aware tag?
Would it be difficult to add 1 or 2 layers of nesting to the main panel?
The answer to that would almost certainly be
yes. MusicBee already has to deal with an almost overwhelming combination of nested album/artist/album artist layouts without a potential second level of albums too.
I would use this a bit myself, but I think the amount of work required compared to how useful it is for most users means it's highly unlikely it would be implemented unless MusicBee undergoes a major rewrite. Moreso when there's a fairly simple workaround already.
But Steven may surprise us...
Bee excellent to each other...