Panning for gold
There is a function on the majority of mixers called Pan. Pan/Panning is controlled by the 'pan pots' which have an L on the left and the R on the right, with center being in the middle. This allows you to decide which side of a stereo track you want a signal to go too. Some mixers only record a stereo track and most cameras have 2 inputs that can be assigned as Left or Right.
Why panning for gold?
Ok, the title can be confusing. By panning for gold I am referring to separating different audio signals so you can keep them clean from each other. If you have a boom and a radio mic on someone and you had the panning set to center (in the middle of L and R) both of the signals would go to the left and the right of the stereo file. This would sound strange if you had them both in the mix because of phasing issues from the different distances the two microphones from the same source. Along with that, if one of the microphones was to become distorted or hit, you would then have unusable sound even though the other source is fine. This is why we pan. It is standard practice to have a boom microphone on the Left (hard panned left so non of the signal goes to the right channel) along with a radio microphone on the right (again hard panned)
But I have more than 2 sources!
When you have more than 2 input sources (2 types of microphones) like on the Sound Devices 302 which has 3. You will have 3 inputs but still only 2 outputs. This is where you mix, using your faders. Lets use the example of one boom mic and 2 radio mics on 2 actors talking in a scene over a table. You will have the script in front of you and your boom operator will have found the best placement for swinging the microphone to capture all the lines of dialogue. You hard pan the boom mic to the Left channel and the 2 radio mics to the Right. As the dialogue happens you use your faders to mix the 2 radio mic signals, focusing on having the fader up for whoever is talking.
Important note about stereo:
Location Sound is captured with mono microphones and even if something is recording to a stereo track (from a mono source) it does not make it true stereo. If you record 1 source in Center and then playback, you will equally play the source through both speakers giving the illusion of center stereo. This is known as phantom stereo. And there you have it. Panning for gold standard audio.
The Half Power Point of -3dB
The half power point essentially establishes that a -3dB difference is half the power of any frequency played through a speaker. A mono signal played through 1 speaker is going to be 100% volume, but when played through 2 speakers you have double the speakers so essentially only need half the power from both to perceive no change in level.