To exempt register boxes installed in floors directly over pipeless furnaces from the requirements of Sentences 3.6.5.6.(2) to (4), which would otherwise require certain clearances between the register boxes and combustible material, if certain conditions are met.
This is to limit the probability that combustible material will be ignited by the register boxes, which could lead to the spread of fire to other parts of the building, which could lead to harm to persons.