Hazardous area is classified into 3 classes
Class I
Locations in which flammable gases or vapors are or may be present.
Class II
Locations which are hazardous because of the presence of combustible dust.
Class III
Locations where hazardous conditions exists because of ignitable fibers or flyings.
Within each class NEC recognizes two divisions. Regarding class1.
Division1:-
The criteria for these locations is that they are likely to have flammable gases or vapors present under normal conditions.
Division 2:
The criteria for these locations are that they are likely to have flammable gases or vapors present only under abnormal conditions.