The large left knob (COM/VLOC) is used to tune the MHz value of the standby frequency for the COM or VLOC radio, depending on which one is selected with the tuning cursor.
The small left knob (COM/VLOC) is used to tune the KHz value of the standby frequency for the COM or VLOC radio, depending on which one is selected with the tuning cursor. Click in the middle of the knob to toggle the tuning cursor between the COM and VLOC frequency fields.