The "?" means the command was not understood or it is not applicable for the slice it was sent to.
All VFO "B" commands will return a "?" unless the radio has been put into SPLIT mode by issuing the ZZSW1: command. When in split mode, the radio automatically create the VFO B slice (you cannot create it manually). And all VFO B CAT commands sent to the VFO A (parent) slice will respond appropriately.
Programs that support split only have one CAT port, so both VFO A and VFO B CAT commands are sent to one and only one CAT port.
To take the radio out of split, issue the ZZSW0; command.