5.1 surround mix
All modern cinemas use 6 channels of audio, and we want to use all of them for the commercials too. This is why we demand usage of a 5.1 mix. If we only get stereo, we will create a 5.1 audio version. This is not the same quality as a 5.1 surround mix that was created in a sound studio.
The 6 channels are L/R/C/LFE/Ls/Rs (Left/Right/Center/Low Freq/Left Surround/Right Surround). This is what a correct 5.1 mix looks like:
and this will not be accepted: