While taking the stairs during a fire drill, I noticed that door to some floors were labelled cross over floor. Later I went to one of these floors, but didn't notice anything different. My searches so far have proved fruitless (I keep getting suggestions for crossing the floor).
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It is a fire safety measure. Here is a reference. Basically, it means that in tall enough buildings, there must be an unlocked door to each staircase every few floors. This ensures that somebody in the building only ever needs to walk up or down a few flights of stairs before they can "cross over" to another exit.
A crossover floor or re-entry floor as they are sometimes referred to are floors that allow you to exit one stairwell and proceed through a common corridor (normally the elevator lobby) to the other stairwell. They are normally 5 floors apart (but this is not a perfect world, so sometimes they may be less or more than the normal 5 floors). The use of these floors (crossover/re-entry) is in case you run into a problem while evacuating a building (ie smoke, fire, water, stairwell blocked). You can exit the stairwell as they have to be unlocked and they will provide you access to the other stairwell so that you can continue to evacuate.