Problem: Tags for some demo tracks (e.g., RATM XX CD2) do not clearly distinguish demos from their commercial release counterparts.
I can’t find a MusicBrainz definition of a demo or how they should be tagged. All I found was this derailed forum post.
Is there a tagging convention for demos?
Thanks!
In short, no.
Apart from a recording disambiguation, like on the release you linked (where it says (demo)), we store almost all things in MB as they appear on the release - according to the images at Discogs and Amazon these tracks are currently entered correctly.
If you want to store things differently to the release in your files/tags, you have to make use of relationships or other data that’s available- you could use a script to add track disambiguations to your track title afaik?
Thanks, I think that cleared it up. So using only MB data, sometimes the fact that a track is a demo can be gleaned from the album title, other times it can be determined from recording disambiguation, sometimes it may be in the track title, and there are likely also cases where there is no indication a track is a demo. I guess that will work for 90% of my use cases.
I thought of MB as systematic, uniform and quasi-comprehensive community-driven tagging which made use of human knowledge, but I guess it’s more like album cover data mining.
Hmm, in the past we ‘standardized’ more stuff, but of course that has its own issues - who decides what’s the correct ‘standard’ for everyone? what is artist intent worth and when does it apply? are we losing precise and unique data that may never be regained (as opposed to standardizing, which can always be done later)?
Luckily there’s still plenty of room for human knowledge, recording credits and relationships can always be set to ‘correct’ no matter the release credits, or more information added that perhaps isn’t on a release.
In any case, eventually (soon??) this will solve all our problems, a new age will arise, and peace will descend upon the earth 
https://tickets.metabrainz.org/browse/MBS-4501
p.s. perhaps consider adding the disambiguation to your tags, I think it will solve your issue?
A volunteer here:
My understanding is that the aim goes beyond that to being the encyclopedia of recorded music.
And the graphic design conveys a “we’ve got it all handled” nonchalance so very well.
The reality is that Musicbrainz is still developing. There are many bare spots.
And that you are a contributor to Musicbrainz development (with this thread).
If you’ve got the energy please put your case for why distinguishing demos is important.
And if you have db know-how then describe how you think they should be captured.