Maybe I wasn't clear, I'm referring to situations where you would typically use FMC or LFMC, such as at a motor junction box. I'm not aware of any liquid tight flexible conduit products that are constructed "without an overall nonmetallic covering".
There is non that I am aware of other than pvc covering and that does not comply. I have yet to find any product with the verbiage used. What does this actually refer to, impervious to what? The purpose would to prevent smoke being emitted into the air, but standard AC cable I do not believe it to be impervious
(B) Ducts Specifically Fabricated for Environmental Air. Only wiring methods consisting of Type MI cable, Type
MC cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering shall be installed in ducts specifically fabricated to transport environmental air. Flexible metal conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to connect physically adjustable equipment and devices permitted to be in these fabricated ducts. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices shall be permitted within such ducts only if necessary for the direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type luminaires shall be permitted.