The ID3 specification, which is the standard for MP3 metadata, does not define a tag called “AlbumArtist,” which was a huge oversight. Way back when WinAmp was the premier music playing app, they decided to use the TPE2 tag, which ID3 defined as “Band/orchestra/accompaniment,” as AlbumArtist, and pretty much everybody has followed suit ever since, including Plex.
So for Plex and most other apps, “Album Artist” maps to TPE2, while “Artist,” which is the artist for an individual track, is TPE1 (defined as “Lead performer(s)/Soloist(s)”). I don’t know if what you called “Contributing Artist” is the same as what ID3 calls “Artist.” I suspect it is, but “Contributing Artist” is not a phrase used in the ID3 specification, so I’m not certain.
For Plex, leaving “Album Artist” blank is likely to cause problems eventually. There are several threads in these forums that were solved by populating “Album Artist” correctly.
As for disc numbers, Plex used to not like “1/2,” but I’m told that’s not the case anymore. Personally, I think the “/2” is redundant. If each disc has the correct disc number by itself, Plex will figure out how many discs there are in the set. But it shouldn’t be a problem either way, as long as you are consistent within any album.
This How-To has a little more detail about how Plex uses tags. You may find it helpful:
[HowTo] Configure Plex to use embedded metadata (music)
When an album is tagged with “AlbumArtist” = “Various Artists” and each track has “Artist” = [individual track artist], Plex will display it like this:
