Other common industrial/agricultural set ups include a 6 or 7 pin round and a 4 wire flat. The round runs the tail lights and Electric brakes etc. while the flat provides accessory power for things like acc. lights, a winch, a hydrulic pump (for dump bed trailers), a Hoist, etc. The round is wired traditionally so any vehicle can pull it down the road and have tail lights, Trailer brakes etc. but unless you have the 4 wire flat too and wired like the trailer then you will not beable to operate any of the accessories on it.
But wether it is 4 flat, 6 round, or 7 round your tail lights (AKA turn/brake/park lights) only run off of 3 wires.
The reason that Santa is always happy, is because he knows where all the naughty girls live!