The boxes work by resisting heat damage from fire. However, you need to understand that the fireboxes are not 100% safe since they protect contents up to a certain period. Fireproof boxes have different fire resistance ratings such as one hour, two hours, 2.5 hours and so on. This is the duration in which the boxes can resist fire damage.
When buying a fireproof document box, you should look at the rating. A one hour or less rated box will not be useful in times of fire exposure since by the time you manage to put off the fire, the box will be damaged as putting off fire may take more than one hour.
Also, many fireproof boxes can also withstand water damage for a certain period. In case flooding occurs, you can be sure that your documents are still safe for several hours. An ordinary waterproof document safe can keep your documents dry for at least 12 hours.