Can someone turn this into english: "when supply or return ducts pass through the floor, a 1/4 inch thick nocombustible resilient gasket must be used between the duct, unit and floor."

I'm also trying to figure out why most of the jobs I see don't have this gasket.

Why wouldn't register boots in floors/walls/celings need a gasket like this or does mastic sealant qualify.

None of the uppers I have talked to can explain this very good. Thanks for any help.