Class 1 definition come from the hazardous area classifications of NEC500 standard which classifies flammable substances into fours types, “M” for methane under mine, “class 1” for hazardous gas and vapor, “class 2” for hazardous dust and “class 3” for hazardous fiber, thus class 1 light fitting is explosion proof lighting specially used for hazardous locations which hazardous gas and vapor present normally and abnormally, but class 1 light fittings have two types, class 1 div 1 light fitting and class 1 div 2 light fitting. If the hazardous gas and vapor produce normally in the environment, you need to use class 1 div 1 light fitting, if the hazardous gas and vapor produce abnormally in the environment, you need to use class 1 div 2 light fitting.