Good question George
Mode A has the ability also to handle telemetry etc (basically bi-directional)
Mode B is the "old mode" as I understand it. It is a one-way communication protocol (which uses the (almost) same protocol as other radios.
When configured to Mode B the PWM outputs are disabled on the receiver, why it is so, I have no clue !!
In Mode A all the PWM outputs continue to function
I'm by NO means a "JR XBus" expert, I have just seen this behavior as I'm designing add-on equipment for our jets and needed to be able to use both Futaba SBUS2 and JR XBus system
(so take what I write with a grain of salt
)