From the ALSA wiki
A ctl device ("control-device") on a soundcard is used to help the user control several aspects of the soundcard's behaviour. The main use is to control the mixer. A ctl device has several controls which can be controlled using alsamixer or other similar programs.
The ctl device can often also be used to toggle digital inputs/outputs, etc.
See proc asound
for information about the ctl device(s) on your soundcard(s). Each ctl
device can have several controls like "Master" or "PCM". You need a
mixer program to adjust these controls. They are not available via the
/proc interface as far as i know.
Retrieved from "http://alsa.opensrc.org/Ctl-device"