The ID card is an essential item that can be used in Barotrauma.
Whenever a character spawns, they are given an ID card by default. Depending on the job the character has, they will be given one with a unique clearance level specific to that job. The cards are primarily used for opening doors on the submarine which are inaccessible without proper clearance.
The captain has superior access to the rest of the jobs, and is capable of accessing any room, excluding the private cabins of the other crew members. It is however possible to steal the captain's ID card from them once they are dead.