When to undercut a door?

A door is undercut sometimes in a very small room such as a closet to allow airflow in or out of the room without putting in ductwork and air vents into the room. Sometimes a grille is put in the door to serve the same purpose. Undercuts and grilles in doors are not always allowed in rooms like utility rooms or storage rooms because the room might have a fire rating and the door would then need to be fire rated. If an undercut is allowed make sure that the door threshold doesn't then get installed and cut off any air flow that might have been needed with the undercut.