It depends ....
As with all things Marlin lol. If you have configured your RAMPS in Configuration.h as 33 this tells Marlin that you, potentially, want a hot end heater, heated bed and one fan. If you chose 35 then you could have hot end heater and two fans. Let's assume you used MOTHERBOARD == 33. D9 would be the fan output (it supplies 12V when activated and you need to be conscious of the +ve & -ve polarity) D10 would be your hot end, polarity doesn't matter and D8 would be your heated bed, again polarity doesn't matter.
Andy
As with all things Marlin lol. If you have configured your RAMPS in Configuration.h as 33 this tells Marlin that you, potentially, want a hot end heater, heated bed and one fan. If you chose 35 then you could have hot end heater and two fans. Let's assume you used MOTHERBOARD == 33. D9 would be the fan output (it supplies 12V when activated and you need to be conscious of the +ve & -ve polarity) D10 would be your hot end, polarity doesn't matter and D8 would be your heated bed, again polarity doesn't matter.
Andy