Kind of like calling FRTW noncombustible.....it doesn't really burn, but it is not noncombustible either...
703.5 Noncombustibility tests. The tests indicated in Sections
703.5.1 and 703.5.2 shall serve as criteria for acceptance
of building materials as set forth in Sections 602.2,
602.3 and 602.4 in Type I, II, III and IV construction. The
term “noncombustible” does not apply to the flame spread
characteristics of interior finish or trim materials. A material
shall not be classified as a noncombustible building construction
material if it is subject to an increase in combustibility or
flame spread beyond the limitations herein established
through the effects of age, moisture or other atmospheric conditions.
703.5.1 Elementary materials. Materials required to be
noncombustible shall be tested in accordance with ASTM
E136.
703.5.2 Composite materials. Materials having a structural
base of noncombustible material as determined in
accordance with Section 703.5.1 with a surfacing not more
than 0.125 inch (3.18 mm) thick that has a flame spread
index not greater than 50 when tested in accordance with
ASTM E84 or UL 723 shall be acceptable as noncombustible
materials.