what is the purpose of the plug on the IFI breaker pannel?

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.

Direct link to the Forum

There is also much discussion about it on CD from last year here

Hope that answers the question