Yep, you are right, Dave...having driven these alot, that's exactly what it says. The socket next to it, "spotter circuit", is used to move the locomotive when the unit is not running. You plug a box into it that has a push button, set the isolation switch to "jog" and when you press the button it energizes the first traction motor allowing the unit to move under battery power.
It's used to hostle locomotives into service buildings without having to have the diesel engine running, thus eliminating the smog and diesel fumes dangers.