WHAT ARE DOGTAGS?

A dog tag is the informal name given to the identification tags worn by military personnel, because of their resemblance to actual dog tags. The tag is primarily used for the identification of dead and wounded and essential basic medical information, such as blood type and history of inoculations, along with providing religious preference. Dog tags are usually fabricated from a corrosion-resistant metal although, they have been made from whatever was available.

It may contain two copies of the information and be designed to break easily into two pieces. Other nations use two identical tags. This allows half the tag to be collected for notification while the other half remains with the body when battle conditions do not allow the body to be immediately recovered.

Source wikia.org