[jjd-uk]

I have not read the whole of the thread so apologies if this has been mentioned, but regarding determining what is a Set, why not use the TPOS tag? that's it's role according to id3v2.3.0 see http://id3.org/id3v2.3.0#Declared_ID3v2_frames where it's defined as 4.2.1 TPOS [#TPOS Part of a set] so if TPOS is filled create a Set.

the problem with TPOS is (as I understand it) it will likely apply to double cd releases that arent "box sets" as such but just double cd releases. 

For instance, Revelling/Reckoning would likely be tagged as TPOS 1/2 2/2 

https://musicbrainz.org/release/4f5ccefc...429faa1c83

Have read a bit more and realised I perhaps have missed the point of this, I had assume it would be a general sets feature so used for displaying all albums with multiple discs, but it seems this has perhaps a more specific target of where several albums are grouped within one package, being restrictive like that seems like a wasted opportunity to me.

@jjd-uk TPOS contains the disc number and total discs for a multi-disc album.  It's impossible to determine from that alone if the album is indeed a box-set or just a multi-disc compilation or an anthology.

Why not make a node for any album containing multiple discs? then for those that want what just a box sets a filter can be set based on DISCSUBTITLE perhaps controlled by a setting.