The large knob is the primary stereo gain control, similar to the gain potentiometer of the 9100 except rather than being infinitely variable it features detents corresponding to specific gain intervals. It is a gross, over-all gain control, the easily adjustable one you will change when recording louder or quieter events.
The two trimmer pots are infinitely variable and are used to precisely gain match the two channels - useful when the microphones have slightly different sensitivities. The posts can also be used in combination to alter the overall gain range through the preamp, changing the gain associated with the detents of the large. They range from muted signal (fully counterclockwise) to full gain (fully clockwise).