I wasn't able to find a thread, as fast as I would like to using the search, which already dealt with the following, please don't shoot me if I overlook it ;-)

What would be the string for the tag set shuffle to be able to have the shuffle plugin do the following (if it's possible in the first place):
- Act like the shuffel album 2 mode (start at the first song of the album and start with a new albums if all tracks have been played of the first album)
- Dealing with multiple discs in a release so it's starts playing with the first song of the first disc and when that disc is played completely it should start with the first song of the second disc.

I have added PARTINSET (ID3v2 tags) and DISCNUMBER (all other tags) tags to the songs to specify which disc it is.
I have not added the DISC tag (which foo_shuffle does use) since that one isn't an "official" tag.

Not sure what you meant by official tag, but whatever.

Copy values from DISCNUMBER to DISC through the Masstagger, and foo_shuffle will play multiple-disc albums in full order.

No, that's just what I would like to prevent. I want to keep PARTINSET and DISCNUMBER because those are considered to be more generally used compared to DISC. Therefore I figured the tag set shuffle mode might be able to do the trick though I'm not sure what the string should be like to have the same behaviour as the Shuffle Album 2 mode plus using the extra disc tags. Is this possible?