When looking through the manual, I usually open the PDF file found in /bb, and then search it with CTRL+F. When I did that with MGX, the manual had a small section about MGX files on page 179 (this is the 2015 Manual).
If one loads a multitrack MIDI file into BIAB -- say a MIDI file that was found on the internet -- the file will load all tracks onto either the Melody track or the Soloist track.
When this is saved as a BIAB file, the file will have an MGX extension.
In other words, if your file has .MGX, it means that it contains a multitrack MIDI file on the Melody or Soloist track.
Normal BIAB .MGU files are a single MIDI track. On the top menu, if you select "Melody | Track Type", you'll see what I mean.
LINKS TO MY BIAB/RB SONGS