Good question. A pin description of the Reface mini-midi cable would be good.
Yamaha should put such a description in the appendix of the Reface manuals.

or post it here in this forum 😀

You should contact Yamaha Service - they will be able to best tell you about the breakout cable. It is not outlined in any of the User documentation.

Okay, any specific email address or place I should contact them? Thanks for your help!

The one closest to you. Yamaha addresses and phone contacts are list in the Owner's Manual by country.

Please see Reface MIDI pin-out

I used this information to successfully create a "crossover" cable to connect two reface's with one cable (this cable send the MIDI OUT of each Reface to the MIDI IN of the other).

Orient the cable so that you are looking at the 6 pins (mini-DIN male end), with the little arrowhead at 12 o'clock. This is the orientation in which you can plug the cable into the reface. Starting at 12 o'clock (where there is no pin) and going clockwise, the pins are numbers as follows: 6, 4, 2, 1, 3, 5.

Pins 6, 4, and 2 (the 3 on on your right) are MIDI OUT
Pins 1, 3, and 5 (the 3 on your left) are MIDI IN

The specific connections are as follows:

Pin 1: MIDI IN, Ground via decoupling capacitor
Pin 2: MIDI OUT, Ground
Pin 3: MIDI IN, LED cathode (-)
Pin 4: MIDI OUT, TX serial data
Pin 5: MIDI IN, LED anode (+)
Pin 6: MIDI OUT, pull-up to 3.3V

To make my crossover cable, I bought a kenable 6-pin audio video cable, cut it in half, and reconnected the wires so that the pins on one end were connected to the pins on the other end as follows:

Pin 6 to Pin 5
Pin 4 to Pin 3
Pin 2 to Pin 1
Pin 1 to Pin 2
Pin 3 to Pin 4
Pin 5 to Pin 6

This replaces 4 cables (the 2 breakout cables, and the 2 standard MIDI cables that connect them to each other).

Good luck!

Thank you very much, Peter! That was some tough info to track down. I got sidetracked with my Reface Patch Bank project, but will definitely circle back to this. Thanks again!