They're actually documented inside MODX itself. Since you are able to configure the CC values for various controllers (A.SW1, A.SW2 included) - you can look at these screens to see how they are set.

If you press the Performance name, then press "Edit" on the screen when the left-hand popup appears, then "Control" -> "Control Number", you'll see the settings.

The screen looks like this:

Rev A0 of the Data List, page 184 ( https://usa.yamaha.com/files/download/other_assets/2/1192592/modx_en_dl_a0.pdf ) shows the default values for controllers:

BREATH CONTROLLER 2
FOOT CONTROLLER 1 11
FOOT CONTROLLER 2 Super Knob
ASSIGNABLE KNOB 1 17
ASSIGNABLE KNOB 2 18
ASSIGNABLE KNOB 3 19
ASSIGNABLE KNOB 4 20
ASSIGNABLE KNOB 5 21
ASSIGNABLE KNOB 6 22
ASSIGNABLE KNOB 7 23
ASSIGNABLE KNOB 8 24
RIBBON CONTROLLER 16
ASSIGNABLE FUNCTION 1 86
ASSIGNABLE FUNCTION 2 87
MOTION SEQ HOLD 88
MOTION SEQ TRIGGER 89
FOOT SWITCH Arp SW

Assignable Function 1 (AF1, or A.SW1) = CC 86
Assignable Function 2 (AF2, or A.SW2) = CC 87

Also note that Control ARPs can set/clear A.SW1 and/or A.SW2. There are built-in arpeggios that already do this. Therefore, one option for controlling these switches is to use ARPs as well.

Wow - this MODX is awesome! (As are you, thx J!)