A/B switch toggles between 2 and 6 button mode. And you are correct-- only buttons I and II have turbo adjustment switches. I can think of exactly zero instances where you'd ever need turbo functionality on buttons III-VI.
The Avenue 6 is a really great pad, but they should have added an option for 3-button mode in addition to 2 and 6. That would have been the ultimate Turbo controller right there.