From the manual:
12. EXP 1/2, FS 7/8 Connect an expression pedal here to adjust a wide vari-
ety of parameters.* With a Y-cable (tip-ring-sleeve 1/4" jack, split to two tip-sleeve
1/4" jacks), two expression pedals can be connected. Alternatively, you can add
one (or with a Y-cable, two) external footswitches to access additional stomps,
presets, snapshots, etc. (FS7=tip, FS8=ring). To set this jack’s function(s), see
page 57. By default, this jack is set to accommodate a Mission SP1-L6H Line 6
Expression Pedal, which has a toe switch that toggles between EXP 1 (Wah/Pitch
or Poly Wham) and EXP 2 (Volume/Pan). For external footswitches, the momentary
(unlatched) type should be used.
*NOTE: It is recommended to connect to this EXP1/2, FS 7/8 input while the device is
powered off to avoid toggling the current FS 7/8 setting.