Different baud rates

Arduino --com a--> RN42 module --com b---> slave module --com c--> MIDI commercial product

hmmm, I've read thru that data sheets a few times, and always thought there was just one baud rate to set.
So, if my Master BT module is the RN-42

On page 11 it says, "Example: SU,57 sets the baudrate to 57600 baud."

Is this changing 'com a' or 'com b'?

I thought it would be changing 'com a', and then on my Arduino I would match it with 'Serial.begin(57600);'