BW specifies the bandwidths to use for the scan. One value can be given for each baseband channel. Any unset values will be set equal to the first so it is usually only necessary to specify one. Because of details of how the defaulting works, once more than one channel is explicitly set to a non-zero value (rather than just defaulting to the first), be sure to set that channel explicitly again, or it will retain the previous value. To get back to the default, explicitly set the bandwidth to zero. Note that, once upon a time, the sign of the bandwidth indicated the sideband, but that is no longer true.
Some (all?) of the digital backend systems, including the RDBE, cannot oversample. Plus the software job in SCHED to allow the sample rate to track the input bandwidth is looking a bit serious - the sample rate gets entangled with disk management etc. So, at least for now (written in Dec 2013), it will be necessary to make a new setup file to use a different bandwidth for most observations.
Note that new parameter CRDBW can be used to set the VLBA legacy system BBCs to a bandwidth when using the RDBE, which, with the PFB personality, cannot be set to narrow bandwidths. This is useful for reference pointing. Since the legacy system can oversample, CRDBW can be used without a new setup file.