CRDSETCH is a list of setup file baseband channels to use for many channel dependent parameters such as IF channel, polarization, etc when setting up the channels for the VLBA legacy system. There can be as many entries as there are legacy system channels. As of early 2014 when 4 BBCs are being removed, that number is only 4. The effect is only on the crd files, not the Vex file.
This parameter can be very important when using the S/X system on the VLBA. The new VLBA control system will know that both S and X band receivers are in use and will set the hardware accordingly. Critically, it will or will not deploy the ellipsoidal reflector over the X band receiver. If the channels sent to the legacy control computer (the crd files) do not contain any S band channels, then that system will think this is an X band only observation. It has control of the subreflector so it will point the subreflector at the X band receiver. But that optical path is now blocked by the ellipsoid. So no source data will get through. However all other parameters like system temperature and pulse cal will look ok. You end up with hard-to-diagnose lack of fringes. This situation can, and has (guess how we know about this!) happened when there are many X band channels and only a few S band channels as is likely when using the RDBE_PFB.