LCHAN specifies the IFCHAN to use if IFCHAN='L' as in LCP. RCHAN serves the same purpose for RCP. If an invalid argument is used, SCHED will complain that IFCHAN is bad. This allows IFCHAN to be specified by polarization which is more meaningful to the observer. It also makes it easier to deal with antennas, such as the VLA, that might have different IF assignments than the VLBA.