Hazardous locations are defined as "premises, buildings or parts thereof where fire or explosion hazards may exist due to the presence of flammable gases or vapors, flammable liquids, combustible dusts, or easily ignitable fibers or flyings".
In North America, hazardous locations are separated into three “Classes” or types based on the explosive characteristics of the materials. The Classes or type of material is further separated into “Divisions” or “Zones” based on the risk of fire or explosion that the material poses.
In other parts of the world, areas containing potentially explosive atmospheres are dealt with using a “Zone System”. Zones are based predominantly on the International Electrotechnical Commission (IEC) and European Committee for Electrotechnical Standardization (CENELEC) standards.
Whereas North America deals with multiple types of hazardous atmospheres, the North American Zone System presently addresses only flammable gases and vapors which is the equivalent to North America’s Class I locations. The most significant difference in the Zone system is that the level of hazard probability is divided into three Zones as oppose to two Divisions.
UL 1203 specifically addresses explosion-proof and dust-ignition-proof electrical equipment for installation and use in hazardous (classified) locations, Class I, Division 1, Groups A, B, C, and D, and Class II, Division 1, Groups E, F, and G, in accordance with the National Electrical Code, NFPA 70. It also covers explosion-proof electrical equipment for installation and use in Class I, Zone 1, Groups IIA, IIB, and IIC hazardous (classified) locations and dust-ignition-proof equipment for use in Zone 20, 21, and 22 locations.
There are a plethora of white papers readily available on-line which go into much greater detail; a few of these are linked below: