Number Lock

Number Locks are one of the more heavily guarded locks. As they were used to get into areas not ment for the general public (such as private quarters or more extreme, the torture chamber in Neptune's Bounty). When you come across one, it’s locked and needs a code to open it. The codes can be obtained by either Audio Diaries or written somewhere in Rapture. Most codes are given before encountering a safe, but some are given sometimes as far ahead as 4 levels into the game from where you found the lock. Once hacked or opened, these locks provide acces to rooms with either audio diaries, gene tonics, consumables or ammunition.

Trivia
Langford's safe has a number lock.