In ID3 ratings are stored in POPM frames. And there is an email field, for which the basic idea was that a file can contain ratings for different users. But this is not how it was used in practice, usually players just write their own specific label there. E.g. MusicBee uses “MusicBee” and Windows Media Player uses “Windows Media Player 9 Series” (which many other players also use for compatibility). No idea what Media Monkey does.