Extra receptacle on DC to DC Converter

This is the second time that I've replaced the auxiliary battery, and I guess I didn't notice this the first time. My understanding is that the black box mounted over the auxiliary battery (in a Gen 2 Prius) is a DC to DC converter. I have three plugs that had to be disconnected to remove it, one the main power connector, one a voltage sensing connector, and the third one plugs into the braking system capacitor next to the battery. While putting it all back together, I noticed that the DC to DC converter has a third receptacle, which isn't used. I don't have any extra plugs back there, and I'm pretty sure that I just disconnected two plugs from that box? What is this third receptacle used for?

 

Thank you for the reply

 

I went back to the other site that I had been looking at (sorry, I'm too new to provide a link) and realized that I had misread the comment about the sensing wire. I see now that the DC to DC converter is up front in the engine compartment and in order to regulate the voltage of the auxiliary battery, there is a wire that runs from the fuse block on top of the auxiliary battery up to the front of the car. This voltage sensing wire is protected by the 5 amp fuse in the fuse block. The main power lead is protected by the much larger fuse (120 amp). The extra receptacle has a nice large spade, but must have been for some auxiliary power off the battery, maybe for an audio system.