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They are code locks that you can use to go in locked doors/rooms. In BioShock, you can either hack the locks/Key pads, or you can find the code (almost always given in an audio diary, though sometimes written on a wall or something), though you have to memorize it. The audio diaries are usually found relatively close to the doors, though sometimes they are far away, but in that case, there is usually only one door on that level.

In Bioshock 2, once the code is found, it is shown while the code lock interface is up.

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