FB2 are the repeater outputs. That's what you'd want to listen to.
TX frequency depends on what point of view you are using.
The repeaters output/transmit is on the lower frequency, and the input/receive is on the higher frequency. (specific to this system, others may vary, but on UHF, that's ~usually~ the way it works).
The portable/mobile radio will transmit on the higher frequency (repeater input) and listen on the lower frequency (repeater output).
Trying to listen to the MO or FX1's may be difficult. You may only hear one side of the conversation, and you'd have to be in a good location to hear them.