ummm.. let's see. Say I have a C and want to pitch-shift it to E. Given the fact that we are in equal temperament, I need to program a 400 cent shift (100 cents gives a semitone). Now if I play a D, the pitch shifter gives again a 400 cent shift, thus a F# and everyone is happy. What is then a diatonic shifter? How does it differ from this kind of pitch-shifting that I have mentioned?
PITCH SHIFTING simply raises/lowers the pitch of a *fixed* set of cents.
So if you set +400 cents, any note will be up_shifted by a major third. It has no relationships with diatonic scales. It sets only a fixed interval.
DIATONIC PITCH SHIFTING assumes you play a tune in a scale/tonality and you need notes to be shifted within THAT scale. So you set scale ROOT, scale TYPE and interval. If you choose C Major scale and third, every note played in that scale will be up_shifted a DIATONIC third, correctly staying in the scale.