12mm refers to the spacing between the conductors.  This connector style was designed as a home television antenna connector, in Germany.  However, the contact area is sufficient to carry a great deal of current.   Between that and ready availability, I suppose we can understand why Widder decided to use it as their connection for electric clothing.  Widder now makes gear for Harley Davidson, so Harley electric clothing also uses this connector.  BMW uses it as well, for some of their gear.  From my limited experience, it appears Widder has standardized on the female connector as the battery side, and as you look down at the screw terminals with the sockets pointed ahead, the hot wire is to the right.