Command neural interface
From Halopedia, the Halo wiki
Command neural interface or CNI implants are implanted in the brain of every UNSC ship commander. In addition to the duties of a standard neural interface, they also have several purposes specific to a ship commander.
Once installed, they cannot be removed without killing the host or doing permanent damage to the host's brain and are therefore permanent. Their main use is the storage of data codes, NAV data, and also for receiving telemetry from outside sources. AIs can access the lace with the owner's consent and retrieve data for use. The owner of the lace can also receive data about the ship they command from onboard AI and subroutines, relayed to the individual officer's personal synchronised comm band. The CNI transponder of the commanding officer is the only thing with clearance for initiating a vessel's auto destruct. Thus, a UNSC ship may only be destroyed if its commander authorizes its destruction, if it is done manually, or if the commander is dead, their interface may be removed and the codes can be fed into the ship's computer by an AI. Although permanent, they can be upgraded for specialized functions.
CNIs are designed so that the classified information stored within them, such as the locations of UNSC colonies, cannot be forcibly extracted by interrogation or torture, and have been proven effective against even the Flood for a limited time.
In the Halo games, the hidden skulls and the Oddball have a command neural interface at the back that can be clearly seen in Theater Mode. Additionally, in Halo 3 on online multiplayer Capture the Flag, one can see command neural interfaces on the skulls atop both teams' flag.