Identification friend or foe

Identification of Friend or Foe is a system used by the UNSC to tag friendly or hostile units for easy identification on a Heads-Up-Display or motion tracker. 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 tracker, while the crosshairs of a HUD turn green when pointed at them. Hostile units show up as red on a motion tracker, and the HUD crosshairs turn red.

In the Halo 2 level, "High Charity", the Master Chief managed to rescue a dozen UNSC Prisoners of War. This was due to Cortana identifying their "IFF Transponder"s, which were still with the Marines.

Most UNSC ships, ranging from civilian Freighters to UNSC Navy Warships, possess IFF transponders displaying their identity, allegiance, and classification