Thanks for answering! I am sure my terminology is wrong. Let me try describing it again. I typed in some chords and picked a realtrack style. I press play and everything works great. Then I right click "guitar" at the top of the screen and select "assign realtracks". Then I select a sax part instead of a guitar part. I close the dialog and press play. The sax plays for the fist bar then stops. The rest of the song (parts?) play fine!
I can try switching back to the original guitar part and and press play and the guitar plays as advertised.
I thought this was part of the power of BIAB, but am I just asking it to do something it is incapable of doing?

Thanks for the help!