The one that a PWM cable can plug into next to the LED. Could it be a remote reset switch?
From the IFI Support Forum:
In the center of the breaker panel, near the LED indicator, there is a three-hole socket that looks like it could accept a PWM wire. Is this socket implemented currently?
This connector should not be used. It provides the same RS-232 data that is present at the DB9 connect.
There is also much discussion about it on CD from last year here
Hope that answers the question