There appears to be a mismatch between how MediaMonkey and MP3TAG handle custom fields on flac and mp3 files. Maybe others also, but I have only tested flac and mp3.
I don't know if this is actually a bug, or if it is, who owns it, so I'm posting this in both MediaMonkey and MP3TAG forums.
In MediaMonkey, the "real name" of the first custom field (for all files, apparently - both mp3 and flac) is SONGS-DB_CUSTOM1. This appears in MP3TAG as an extended field - COMMENT SONGS-DB_CUSTOM1.
In MP3TAG, the same field for a flac file appears as an extended filed with just CUSTOM1.
I have no idea which is "correct," if either. I just want the applications to work together - which means that when I set the field in MP3TAG I would like the same value to appear in MediaMonkey. Of course, I can put both values in the MP3TAG panel, and remember to set both, or set the correct one depending on which type of file I'm looking at. But I use several custom fields to help organize my music, and doing this all of the time would be a hassle.