Identification of Friend or Foe (otherwise known as IFF) is a system used by the UNSC to tag friendly or hostile units for easy identification on a Heads-up display or motion sensor. The equipment of a friendly unit generates a signal on a specific frequency, which is interpreted as a "friendly" signal by the receiver. If a signal is not generated, then unit is tagged as a hostile. Friendly units appear as yellow dots on the motion sensor, while the crosshairs of a HUD turn green when pointed at them. Hostile units show up as red on a motion sensor, and the HUD crosshairs turn red.