POW-r #1 is optimized for low dynamic range program, such as highly compressed pop music
POW-r #2 is optimized for less complex program, such as spoken word
POW-r #3 is optimized for complex, high dynamic range program, such as orchestral performance
These are only basic starting points. There are no “right” and “wrong” settings — optimal performance is entirely dependant upon the source material, production goals, and tons of other artistic criteria.