How is the fan controlled?

Hallo everybody,

I'm a student in internship and my company gave me the task to make a 3D printer.
I'm now figuring out how the elektronics of the 3D printer works but I can't figure out how the fan would be controlled...

I'm using a RAMPS 1.4 (http://reprap.org/wiki/RAMPS_1.4) with four A4988 stepper drivers, an Arduino MEGA 2560 R3 and a RepRapDiscount Full graphic smart controller.
I'm using one extruder on D10 so D9 is not used.

On the site the say: The bottom pair of connectors marked 5A power the stepper drivers and Extruder heater/fan (D9, D10). But i don't understand this, because D9 and D10 are to control the Extruders.. right? And not to control the heater (which i have on D8.. that's right? right?) and the fan.. or can i just connect the fan to D9 and is my problem than solved?

Can anybody help me with this question?

Thanks in advance.

yes, you set your board in the config at 33. and D9 becomes fan control.

Thank you very much cat.farmer!