To exempt a doorway serving a dwelling unit from the requirements of Sentences 3.3.1.3.(8) and (9), which would otherwise require a doorway to the exterior, a public corridor or an exterior passageway, on the basis that there is a second means of egress from the dwelling unit available if the route through the exit stair is impeded, blocked, or untenable.
This is to limit the probability that persons will not have the choice of an alternative egress route in the case where one route is blocked or obstructed in an emergency situation, which could lead to delays in the evacuation or movement of persons to a safe place, which could lead to harm to persons.