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 always considered to be,
and should continue to be, 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 same default applies in the International Mechanical Code.