On the CQ18, channels 1-16 works as you’d expect and if set for USB/SDHC, you can send audio to them from an app and you’ll see levels on the meters 1-16. For 17/18, if you route audio from an app to those, they show up on the Stereo fader, and channels 19/20 show up on the USB fader. I’d expect the CQ20 to be similar, but maybe offset by the additional channels.