There are many presets which exists with a name ending by AB or BA (D4 Classical AB and D4 Classical BA). What is the meaning of these two letters.

B = Bass
A = Alto

So, from left to right, BA = Bass notes on left, Treble on right, and vice-versa.

There are many presets which exists with a name ending by AB or BA (D4 Classical AB and D4 Classical BA). What is the meaning of these two letters.

They describe the stereo panorama:
AB: lower notes tend to the right, higher notes tend to the left
BA: lower notes tend to the left, higher notes tend to the right