If you have ignitable fibers on site, you can choose to use Art. 506 requirements for the zone classification system as an alternative to the division classification system covered in Arts. 500, 502, and 503 for Zones 20, 21, and 22.
Some key concepts/definitions [506.2]:
Dust-ignitionproof refers to enclosing equipment in a manner that keeps sparks and flames contained within the enclosure and keeps dust out.
Dusttight enclosures are constructed to keep out dust (under specified test conditions).
Nonincendive items are not capable of igniting the flammable gas, vapor, or dust mixture. Art. 506 breaks this concept out into four definitions.