In this context, transposition does not refer to changing keys, per se, but the process of moving the pitch of the audio (RealTrack) up or down to match the key of the song.

For example, if you recorded the RealTrack using only D, G, and A chords (and their corresponding variations - minor, sus, 7th, aug, dim, etc), but your song needs to play an E chord, BIAB (after finding out that it doesn't have an E chord) will instead take the D chord and raise the pitch (transpose it) one full step (two semitones) to get the sound of an E chord. Fingering-wise for a guitar, it would be as if the guitarist quickly put on a capo on the 2nd fret and then played the chord as a D (which would sound as an E chord). Clearly a guitarist can't do this in real time, so in reality, the E chord would be played with different fingerings. But in this case, there is no recorded E chord, so the program has to transpose the D chord up to get the right chord sound.

Now, the PGMusic RealTracks actually record a whole bunch more chords than that, but not every variation of every chord. So while maybe a RealTrack has an E7, but not an E9 (but does have a D9), then if you did not turn off transpositions, if your arrangement called for an E9, it would transpose the D9 to an E9 and that's what you would hear. If you turned off transpositions (where it wouldn't use transpositions), then BIAB would instead play the E7 instead of the E9.

Don't know if this helps or muddies the description.

John

Laptop-HP Omen I7 Win11Pro 32GB 2x2TB, 1x4TB SSD
Desktop-ASUS-I7 Win10Pro 32GB 2x1.5TB, 2x2TB, 1x4TB SATA

BB2025/UMC404HD/Casios/Cakewalk/Reaper/Studio One/MixBus/Notion/Finale/Dorico/Noteworthy/NI/Halion/IK

http://www.sus4chord.com