Dog tag
From Halopedia, the Halo wiki
Dog tags are simple identification devices that have been used by military forces for centuries. This practice of identification has continued in the UNSC Defence Force.
Historically, dog tags were simple metal plates attached to necklaces. However, it is likely that with new technologies, the current design of dog tags has changed.
The purpose of the dog tag is to provide information when it is most critical. This includes the soldier's name, service number, and blood type, should they be injured and need medical treatment. All UNSC personnel wear dog tags when on active duty. There are, however, some exceptions. Spartans have never been described as wearing dog tags, as it is possible that they interfere with MJOLNIR and SPI armor. Also, the armour itself acts as a dogtag, containing all info on the spartan wearing it.
Historically, dog tags were 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.
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.
