I don't recall finding any information about what this ID is (confirmed by a 'specification' document I have in the PhatMM project), and I can't remember if I did any actual analysis on what I thought it was.
However, Phathack Media Manager writes the 'mediaid' for the corresponding track from the media table into this field.
By the way, the artist/album/genre .db files aren't used. They were files from early versions of the PhatBox, so unless you are running an old system they're not needed.
If you DO find out what they are (perhaps there's a clue in the Phat4X project?) - please let me know!