Here you can define a specific gain for the stereo mic through the GAIN slider. Pay attention not to overdo, or you'll get clipping. The USE CHANNEL FROM STEREO box below lets you replace the mono mic by the best option from the stereo mic. To illustrate this, let's take the example of a Pakhawaj. The stereo mics are placed on each side of the drum, the mono one in front. As such, the mono mic may capture a more faded sound than the mics set on both sides. Again, this is left to your discretion to choose.