This is simply a correction of an omission in the text. The default requirement for
materials exposed to the airflow in ceiling cavity plenums and raised floor plenums is
that they be noncombustible (which is already shown) or be limited combustible
(which is the option in 4.3.11.2.6.9 and in 4.3.11.5.5.8) or that they meet a flame
spread index of 25 and a smoke developed index of 50 in the ASTM E84/UL 723 test
(and that portion is implied but missing from the text). This change does not affect (of
course) the requirements for materials of construction of the plenum, electrical wires
and cables, optical fiber cables, pneumatic tubing, sprinkler piping, raceways,
discrete electrical products, supplementary materials air ducts or air connectors. It is
simply clarification consistent with the intent