Whew! Aside from everything else, my head is spinning from reading about all the chord progression stuff. The number of alternatives is incredible. I can see that it must take a lot of education, experience, and time to just scratch the surface of this area in music. Then there are novice, rookie, amateurs like me who are not even smart enough to ask a question in this area.

Still, I am very interesting in the OP's question. Due to my lack of knowledge, I can only listen to a particular chord and hear if it sounds "right" to me. It may very well be the "correct" chord choice by BIAB and still not sound like it "fits" to me. That's when I go to the alternative chord choice suggestions in BIAB. Sometimes, the pgm will tell me that there are no other chord alternatives when it just doesn't sound "right" to me (or my wife who plays the flute and piano). In that case I usually just end up using a simple 3 note chord that barely works. I did read though that BIAB may not be able to identify and place chords base upon just a simple single note melody line because it doesn't provide sufficient info. Even, though I know next to nothing about chord progression, this actually makes sense to me. FWIW

Cornet Curmudgeon