To supersede the requirements of Sentence 3.4.3.4.(1) for a minimum headroom clearance of 2 100 mm and permit a relaxation in such clearances for door closers and other devices on doorways on the basis that the small reduction permitted will have a negligible effect on the use of the doorways during a fire situation.
This is to limit the probability that exits will have insufficient doorway heights, which could lead to emergency responders being delayed or ineffective in carrying out their emergency response operations, which could lead to the spread of fire, which could lead to damage to the building.