To supersede the requirements of Sentence 3.1.8.3.(1), which would otherwise require the space between a horizontal service space or other concealed space, and a required vertical fire separation, to be divided at the fire separation, if certain conditions are met.
This is to limit the probability that a fire in one fire compartment will spread to another fire compartment by means of the horizontal service space or concealed space, which could lead to harm to persons in the other compartment.