I have used an ID3 Tag editor to populate the TPUB tag of several albums with the name of the record label. When Plex scans these albums, it does not populate the Record Label field in the album editor.
From ID3 2.3 spec:
TPUB The 'Publisher' frame simply contains the name of the label or publisher.
From tag editor (Kid3)

Plex:

I moved the albums out of my music folders, let Plex scan, then moved them back in, to force Plex to scan them as new.