"MG3" or "SGU" - actually any "MGX" or "SGX" suffix, where the X is either a number or a letter, indicate files that can only be played back in Band in a Box.

These are called, "Melody songfiles" or "Songfiles", the M indicating that a Melody is also present with the tile, the S indicating a Song, with no Melody that the file will play, typically just accompaniment then.


--Mac