The L-C-R Matrix device was created for those who need to generate three unique channels from stereo (2-channel) source material. This is primarily for speaker clusters with distinct Left, Center and Right channels. The only control is a matrix level control (and presets for it).
Matrix Level allows you to choose the amount of Left input subtracted from the new Right output and vice-versa. Essentially, what this does is allows you to choose how the Left and Right inputs interact to form new Left and Right outputs. The Center Channel is simply the sum of the new Left-Right matrix.
These buttons are simply copies of the Matrix Level control with pre-defined settings. There are presets for .316, .5 and .79. The ratios in question are:
of Inverted Left Input
Amount of Original Right Input
of Inverted Right Input
Amount of Original Left Input