from http://bugs.musicbrainz.org/ticket/4180

Musicbrainz releases can have (imprint)labels (as part of release events) and publishers via 'Published by' relationship, currently Picard only handles labels, it could handle both but would need to store in a different field.

Currently for label Picard uses
Mp3 TPUB
Mp4 ----:com.apple.iTunes:LABEL
Wma WM/Publisher
Vorbis LABEL
Apev2 Label

Unfortunately Mp3 and Wma doesnt provide a predefined seperate field for Publisher/Label and label is more important so I suppose the label should be kept in the TPUB,WM/Publisher field but its a bit confusing. Publisher could be added to a TXXX frame for Mp3.

Mp4:Everyone is doing different things at the moment.
MM3 used to use publ atom but now uses reverse dns ("com.apple.iTunes", "ORGANIZATION") for Publisher doesnt have seperate label field.
Winamp uses reverse dns ("com.nullsoft.winamp", "publisher") for Publisher doesnt have seperate label field.
Apple spec doc defines a @pub for publisher and @mak for label, but not aware of it actually be used by anyone.

Apologies for this rambling request, but I think this info is worth recording even if you dont add the extra functionality.