Card key

Snake can acquire Personal Area Network Cards on his mission throughout Shadow Moses Island. He needs them to open certain doors, which require different card numbers. Snake can collect from Cards one through seven, and each card is backwards compatible. This means that if Snake has Key Cards one, two, and three, his Level three Card will open Level one, two, and three doors. The Card works with Snake's own natural electrical field, transmitting data using the salts in his body as the transmission medium.

Trivia
Hideo Kojima, the father of Metal Gear, originally wanted to force the player to use trial-and-error to figure out which Cards worked for which doors. He was inspired by the classic George Romero horror film Dawn of the Dead. In one of the scenes, one of the characters has to go through several keys in order to try and open a door with zombies approaching. Just as the zombies are about to attack, he finally picks out the right key and moves to safety. Kojima wanted to give players this sense of dread and urgency in his game. While he was able to implement this in Metal Gear and Metal Gear 2, he decided not to implement it in further games as it would become too "troublesome".