The ID Card, remembers all the information of its holder. It can be used to track down its holder. They can be used to call people and have many other features. No.6 is the only country to have these devices, that are compulsory to wear.
Anime and Manga Difference
In the anime, the ID is a bracelet which every resident of No.6 has. They can communicate and receive messages through it.
Known People Using Them
Shion - Used to wear on his left wrist. Discarded it when he and Nezumi fled from the Bureau officials. ID Number - QW-55142
Safu - Disabled when taken in for Elyruias sample. ID Number - SSC-000124GJ