UL tests and certifies fire doors. The Building Code requires UL rated fire doors to continue the fire resistance of the adjacent wall. Obviously, the goal of fire rated design assemblies is to prevent the spread of fire in a building.
You should know that fire rated doors must latch and be self-closing (either by a door closer or a release from a fire alarm signal). UL fire rated doors will have a stamp or plate that many Building Officials will verify.
A 3 hour fire rated assembly wall must have at least a Type A (3 hour) fire rated door. A 2 hour fire rated assembly wall in most cases must have a Type B (1 1/2 hour) fire rated door. A 1 hour fire rated assembly wall in most cases must have a Type C (3/4 hour) fire rated door. The fire rating on the doors must be shown on the drawings, but I include the above so you understand the basics of the system.
The allowable amount of glazing (window) in a fire rated door also gets discussed often in the field. Many Owners want to make changes in the door glazing during construction. So you should know the rules of thumb on fire door glazing. 3 hour fire doors can't have glazing, 1 1/2 hour fire doors can have a maximum of 100 square inches of glazing and 3/4 hour fire doors can have a maximum of 1,296 square inches of glazing. Again, there are code variations for certain circumstances, but these are the normal case rules of thumb.