What is this round black thing?

I have gotten tired of going out of the TT, opening the battery box, and turning ON the Blue Sea Disconnect Switch whenever we are boondocking and want to watch a video, then going back to turn the switch back off. I decided to install a disconnect switch just like the one that Airstream installed to disconnect the house batteries from the trailer. I ordered the switch, the cable and a 200 amp Intellitec latching relay. (The inverter is fused at 110 amps so I didn't get the 100 amp model.) When it came, it came with this black disk with a wire and another screw connector. There is no mention of this piece at all. I don't have any idea what it is or how to hook it to the relay. Any help will be appreciated.