Identification friend or foe
From Halopedia, the Halo wiki
Identification friend or foe, or IFF, 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.
All UNSC personnel are implanted with a neural interface containing an IFF transponder. Additionally, most UNSC ships, ranging from civilian freighters to UNSC Navy warships, possess IFF transponders displaying their identity, allegiance, and classification.
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