On Pro Tools, and other similar DAWs such as Logic or Nuendo, the sidechain input within a plugin allows you to select audio on a particular input to trigger the effects of the plugin. Each DAW is different, however, so make sure to read the documentation for sidechain inputs for the one you are using. 

The sidechain input is useful if, for example, you have an HDX, or if you want to use a microphone from the DAW itself (for example, from the Pro Tools hardware). Those devices would not show up on the track’s input dropdown because that list only shows CoreAudio devices. 

On Source-Nexus Mute-On, you can set up the key input/side-chain by clicking the “no key input” message on the plugin and selecting your desired input. 

The “SIDECHAIN INPUT” side of the plugin will immediately start receiving signal if the selected hardware or virtual device selected as the sidechain input is connected. In the example below, we just set up an External Microphone as the sidechain - the plugin’s input is receiving the audio from the “Audio 1” track, while the sidechain input is receiving from “External Microphone”. 

All options available for the plugin’s input are also available for your sidechain input.