Rotors/Discs may be unidirectional but for high performance many have directional vanes and slots. Alcon rotors/discs will come with a Left and Right designation. If you cannot find that or are using a different rotor, his should help you correctly install the disc on your car.
Internal vanes should lean towards the rear of the car. This helps pump more cooling air through the disc. Slots should lean towards the front of the car however the internal vanes take precedence and should drive the direction of the disc, even if that means the slots are not optimal.