BBFILTER sets the baseband channel bandwidth. There is one value for each baseband channel. Any that are not specified will be set equal to the first so in the usual case that all channels have the same bandwidth, only one needs to be set.
For the RDBE systems with the PFB personality (DBE = RDBE_PFB), the bandwidth must be 32 MHz. There are no options. For the DDC personality (DBE = RDBE_DDC), the baseband channel bandwidth can be anywhere between 0.125 and 64 MHz in factor of 2 steps. For the DBBC, the options are the factors of 2 between 0.5 and 16 MHz, with additional options at 32 and 512 MHz.
For the original VLBA system and for MarkIV, the value must be a multiple of 2 times 0.0625 MHz up to a maximum of 16 MHz. Up to 16 values can be accepted. If two or more baseband channels are assigned to the same BBC/sideband, they should have the same BBFILTER assigned. If not the last will probably be used.
If not specified, one half of the SAMPRATE will be used for all channels, if SAMPRATE was specified.
For MkIV DARs in the EVN the 1 MHz, 250 kHz and 62.5 kHz filters are only available as plug-ins and should be avoided if possible. Although the 500kHz and 125kHz filters are standard they are generally only available on the USB of the first few BBCs. Most stations have 4, and are required to obtain up to 6.