Combustible dust is any fine particulate that can ignite and burn rapidly when suspended in air — flour, sugar, grain, wood, most plastics and nearly all metal fines qualify. Settled, it is the fuel for secondary explosions; a layer as thin as 1/32 of an inch across a small fraction of a room can constitute a hazard.
NFPA 652 requires facilities to document these hazards through a Dust Hazard Analysis, and OSHA enforces housekeeping under its Combustible Dust National Emphasis Program. Both point to the same operational question: is the equipment you clean with engineered for the dust you produce? A standard vacuum is an ignition source. An engineered one is a control.