Diki,
The arrangements for the styles (A-C) are standard (follow what you play on the left or full keyboard mode) to generate chords. Arranger D (RIFF) already has chords generated for you so you do not have to play the chords ... focus on your solo, take a short break, look around ...etc. These chords are generated based on a pre-determined table for the type of style in question PLUS your playing sequence/habit - giving you nice chord changes, added sounds, variations within the style (drum, bass, chords ...etc). Pretty much, you are now playing with a "live" band.
It's something to see. I think GLENN T will be able to give you a better explanation from what he and others witnessed at the FLORIDA workshop last month.
Glenn, awaiting your review on that workshop with info. on the RIFF feature for others to see.
Thanks
AJ