I presume that, if I create variations in playing for A & B sub styles in my recording and save the BIAB song with A & B style markings, I will have A & B parts to use in other songs? In other words:

Yes.

1. My recording has 4-to-the-bar guitar strumming for the first 16 bars and a variation with a 3rd beat accented for the next 16 bars.
2. I save the song with the first 16 bars marked as A part and the next 16 bars marked as B part.
3. Loading the User Track into BIAB, will it recognize the A & B parts, or do I need to create 2 separate User Tracks and change the track at the 17th bar in bar settings?

Yes, that is all correct.

Also, the example I listed above had song1.sgu. But you can add as many songs as you want, and they will all be included. For example, you could add My Other Song.sgu (and My Other Song.aiff)

Have Fun!
Peter Gannon
PG Music Inc.