From Halopedia, the Halo wiki
Dog tags are simple identification devices that have been used by military forces for centuries. The UNSC Armed Forces as well as other military organizations, such as the Sedran Colonial Guard, continue to use this form of identification in the 26th century.
Historically, dog tags were simple metal plates attached to necklaces. However, it is likely that with new technology developed since our time, the current design of dog tags was changed. The purpose of the dog tag is to provide information when it is most critical. This includes the soldier's name, rank, service number, and blood type should they be injured and need medical treatment.
Usually, dog tags are removed from a fallen soldier to identify and later report their death, without the need to carry the body, as this would be virtually impossible in a combat situation. For example, in his trek through Installation 04's Library, John-117 found a dead UNSC Marine named Marvin Mobuto, and took his dog tags. Dog tags are supplemented by neural interfaces, which are capable of displaying basic information for identification and additional information on a soldier's vital signs. This information is accessed via a heads-up display. Dog tags are useful in that they do not require a heads up display to access basic identification information. Orbital Drop Shock Troopers, more commonly referred to as ODSTs, have their name, rank, service number and blood type on a sticker on their shoulder, instead of dog tags, although this might simply be redundancy for easier identification.
Gek 'Lhar, a Sangheili commander in Jul 'Mdama's reformed Covenant, collected the dog tags of some of the soldiers he had killed; some of which were Spartans. These dog tag trophies were later recovered by Fireteam Majestic, after Paul DeMarco succeeded in killing the Sangheili.
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