It's the positive side if you have to jump start your engine. If you look on the right side of the engine wall, you will se a knob (sort of) sticking out. This is the negative side for jump starting. If you push (or pull...I forget) the red knob it will reveal a rod that you connect you positive jumper cable.
I overlooked the second picture...you can see a plus symbol behind the red knob and a V12 for 12 volts