That must be a fire door the little sticker says so

So, there is a little sticker which says "fire door keep shut" on a door. This will come as a surprise to many people, but that little sticker does not make that door a fire door. It is fair to say that almost any door which is closed will provide a short term barrier against the passage of smoke and fumes, and prevent the spread of flames, possibly only for a few minutes, which may be enough to allow occupants to escape safely. A correctly manufactured fire resisting door and frame set will protect against the passage of smoke and fumes and flames for a minimum of 30 minutes, sometimes 1 hour, depending on the location and environment.