Analog Control Interface (ACI) is a way to control the value effect parameters with an analog signal, rakarrack-plus use their audio jack ports to measure the signal and send this values like a MIDI value.That means that you can use your MIC as a WahWah pedal, or use a analog volume pedal like a WahWah, or to control whatever parameter you imagine. If you use MIDI Learn up to 20 parameters can be controlled by ACI.
Source is used to select the jack audio input port audio signal.
Gain and Threshold is for control the incoming signal.
Midi Control select the MIDI Control message to send.
Maximum and Minimum control the minimum and maximum value you send.