my keyboard is a mox6!!!

The MOXF and MOX both have a single Foot Controller jack (for a Yamaha FC7 pedal) but their are two Foot Controller devices listed in the UTILTY Mode. Here's what's going on:

In UTILITY, Foot Controller 1 defaults to cc011 Expression; Foot Controller 2 defaults to cc004 Foot Control.
When you plug in an FC7 it will default to being FOOT CONTROLLER 1 and control how loud a sound is (Expression).

The setting you select is global for the instrument - so your selection applies at all times. If you wish the pedal to default to cc004 Foot Control, you would make the setting FC1 = cc004 - this would make it behave as Foot Controller 2.

Why would you choose one or the other of these?
cc011 Expression is a fixed function- it will always affect Volume, period
cc004 Foot Control is flexible- it is not defined what this Control number will affect. You can assign it on a per VOICE basis.

If all you want is Volume, choose FC1 = cc011
If some times you want Volume, but other times you want Wah-Wah, or Panning, or Effect amount, etc., etc, set FC1 = to FC2: it literally becomes the setting for FC2
choose FC1 = cc004 Foot Control.

Now you can program what the pedal does within the Control Set of each VOICE, and store your preference in the USER Bank.

Those who want to program their own custom functions for the pedal would choose the second option, and store custom versions of their Voices in their instrument, as necessary.

Hope that helps.

Just FYI: when you move up to Motif, S-Series, or Montage you are given two physical Foot Controller jacks. That's one of the differences between the MO and its bigger siblings.