The keycard is a plastic card with a digital line that when inserted, opens a keycard lock-operated door. Security clearances (levels) may vary:
Blue/Cyan = Level I
Green* = Level II
Yellow* = Level III
Orange* = Level IV
Red* = Level V
colors marked with * means that they're not currently in-game
It's first and possibly the last appearance is in coop_omega3 where it serves as the first collectible item. It is collected by drilling into a soldier's head, making up a large hole that makes the keycard fall from the head, making it collectible but thus guilty for the one who triggered the event. It can be then used in coop_omega4, there will be no other message triggered than that the player used the keycard, making it unknown if the keycard will be still usable in future parts.