To exempt combustible duct sealant from the requirements of Sentence 3.6.5.1.(1), which would otherwise require the sealant to be noncombustible, if certain conditions are met.
This is to limit the probability that the materials will have inappropriately high flame-spread and smoke-development properties, which could lead to the development of an inappropriate amount of smoke and the spread of fire across the exposed surfaces of the material, which could lead to the spread of fire and smoke to other parts of the building by means of the air duct systems, which could lead to harm to persons.