All of the VLA parameters are, at temporarily, out of use. The old VLA system has been turned off and the EVLA is not yet doing VLBI. The control scheme may change with the EVLA.
VLABW specifies the bandwidth of each of the VLA's 4 IFs (ABCD). This is done with coded digits such that the bandwidth is 50/2**n MHz where n is the digit used. For example, VLABW='0202' specifies that the AC IFs are at 50 MHz bandwidth and the BD IFs are at 12.5 MHz bandwidth. The digits are specified in a 4-character text which is copied to the VLA observe file. The default of '0000' is appropriate for most VLBI observations. The final baseband filters for VLBI at the VLA are in the VLBA DAS and not controlled by the VLA on-line computers.
SCHED can determine the value of this parameter from the frequency catalog if that file includes an entry that covers the desired VLA setup.