Identification friend or foe
From Halopedia, the Halo wiki
Identification friend or foe, or IFF, is a system used by the UNSC and Covenant 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.
Following the fall of the Covenant, the Swords of Sanghelios and Jul 'Mdama's Covenant utilized "semi-feral" associated intelligences to operate Type-56 Shrike turrets. As these constructs had been long abandoned, they could not communicate with modern Sangheili and had to be carefully monitored and locked down prior to activation due to their inability to understand contemporary IFF systems and codeword usage.
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