CRDNCH sets the number of channels to use on the legacy VLBA hardware (BBCs especially) when using the RDBE. The main reason for being concerned about the legacy hardware is that the power levels in the BBCs are used by the old control system to do reference pointing. See also CRDFREQ, CRDDOP, CRDBW, CRDCH1 for controls related to such reference pointing, especially with masers.
CRDNCH is a required parameter when CRDFREQ or CRDDOP are set.
When not using CRDFREQ or CRDDOP, CRDNCH will still set the number of channels used in the crd files controlling the BBCs. If CRDFREQ and CRDDOP are never used, CRDNCH will be set to lesser of 4 (the number of BBCs after April 2014) and the number of setup channels.
When CRDNCH is less than the number of channels in the main setup (typical if using the RDBE_PFB), then the alignment between the main setup channels and the legacy system channels can be set using CRDCH1