DC coupled Inputs 1-4 are summed and sent to the mix out. If a cable is plugged into outs 1-3, that respective channel will be pulled from the mix and become buffered attenuator.
There are three differential amplifiers outputs at the bottom of the module. A simple formula for the differential outputs is this: [diff output] = [ input 1 voltage ] + [ – input 2 voltage ]. Notice that the inputs are added together but the second input is also inverted. As an example, a 5 volt gate at both inputs 1 & 2 will equal 0 volts out of the diff output. Diff 3-4 works the same way, with input 4 being the inverted input. The diff outputs can also be used as simple inverters for channels 2 & 4.
The “diff-diff” is a differential amplifier for the two diff outputs (diff 1-2 is the inverted input to the diff-diff).