[dertyp0815]

In mp3tag, the only available tag, or rather the one that comes closest is "MUSICBRAINZ_ALBUMTYPE". Is that identical to "MUSICBRAINSALBUMRELEASETYPE" or is it possible to just add such tag manually?

The internal names of tags vary with tag format e.g. for FLAC it is RELEASETYPE, ID3 v2.4 (mp3) TXXX:MUSICBRAINZ ALBUM TYPE, and APE uses MUSICBRAINZ_ALBUMTYPE

These various internal names where defined by Musicbrainz, and in Picard (regardless of format type) they get the display label "release type". In Mp3tag I think both the ID3 tags and APE tag get the display label MUSICBRAINZ_ALBUMTYPE, but FLAC does not.

What Mp3tag is showing as MUSICBRAINZ_ALBUMTYPE is the tag you want to set to "boxset". BlackEagle unfortunately made a typo in the post you quoted.

You don't say what format file you are tagging and you don't need this, but yes you can add tags manually using Mp3tag (or you could use Picard instead).

Thanks much! In my case mostly m4a, but as discsubtitle was already set, seems the reason was just that "display as individual CDs" was not activated.