Identification friend or foe

Identification 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 reciever. If a signal is not generated, then unit is tagged as a hostile. Friendly units appear as yellow dots on the moption 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.

IFF at Wikipedia.