If you're running external audio through VM, think Pete Townsend "Who Are You", you need an Envelope Follower and CA provide one. That's sufficient for most uses, certainly from a guitar point of view, but what if you want to process stereo or even multichannel audio. Then you need multiple Envelope Followers or, more conveniently, a polyphonic Envelope Follower.
The Poly-Follower is an Envelope follower with both monophonic and polyphonic audio I/O that will track the RMS level of incoming audio and turn that into a CV envelope signal.
The module provides a Lag control, which determines how quickly the generated envelope responds to short-term changes in the RMS signal level, as well as Scale and Offset controls to manipulate the outgoing CV.
A short demo is available here: https://www.youtube.com/watch?v=RFZz_N8ri6o