I think some of the confusion arises because there are two different functions for transposition.

1. Change the pitch of a whole piece when it's played - to fit with a singer's range, or make use of open strings on a guitar, for example.

2. Change the representation of this piece on the page or screen - to accommodate transposing instruments such as trumpets, saxophones and guitars.

Case 1 is dealt with in BIAB using the key signature button just below the song's title. If you change the key signature using this, you're given the option of transposing the song as well. When BIAB plays this song, it will have changed in pitch.

Case 2 is dealt with through the "Opt." button in the Notation or Chord Window. Click on this, and you'll find you can Transpose the notes you see on a screen by a chosen number of semitones. If you're a trumpeter you'll want it up two semitones so that when you play the trumpet part it matches the backing in the original key, if you're a tenor saxophonist you'll want it up 14 semitones, and if you're a guitarist you'll want it up 12 semitones.

The converse of this is that if you use "Opt." button to transpose up 2 semitones, and then key in the chords from a paper copy written for a trumpet in Bb, BIAB will play the song as if you'd entered the "Concert" version with no Transposition.

In keeping with this, BIAB keeps the original key signature on the key signature button, even though the chords and music on the screen are in the key appropriate for the transposing instrument.

So the ideal way to enter pieces from a book meant for a trumpet (in Bb) goes like this:

1. Set the key signature for the "concert" pitch version of the piece (a tone down from the Bb part).
2. Open up the Notation or Chord Sheet Window, and use the Opt. button to Transpose up a tone.
3. Enter the piece exactly as it appears in the Bb part.
4. Play the piece in BIAB, and play along with the Bb part on your trumpet.

If you want to play the melody on your flute, use Opt.=>Transpose to set the transposition to 0, and if you want to play on your Alto Sax, set the transposition to -3.

Of course, if Phil and his wife want to play together on the same piece, reading their parts from BIAB, we have a problem, as only one part can be displayed at a time. However, Phil already has a Bb part on paper, so BIAB can be set using Opt.=>Transpose at -3 for his wife's alto. They could also print out lead-sheets for an Eb instrument using this setting, and switch to an Opt.=>Transpose value of +2 for Phil to read from the screen.