Neural interface
From Halopedia, the Halo wiki
The Neural Interface is a type of technology used by the UNSC. The standard neural interface has basic functions. It acts as a "friend or foe" indicator, so that radar signatures will pick up the owner's signature and identify it as friendly. This way, the wearer appears as a "yellow" blip on the motion tracker of another soldier's HUD and friendly fire is less likely. It also shows the targeting reticule of the gun chosen.
The neural interface is implanted at the base of the skull and cannot be removed without killing its owner or through sophisticated surgery. The standard interface is implanted in all UNSC military personnel upon activation, but it can be replaced with a more specialized neural lace should the need arise. The standard neural interface also displays the 'owners' name, rank, unit, and other relevant information to UNSC personnel if viewed through a HUD.
Ship commanders receive command neural interfaces, while the Spartans have received the more specialized Spartan Neural Interface.
