"In storage area fire protection, the commodities being stored must be categorized into groups according to their relative combustibility and rate of heat release in the event of a fire. By definition, a 'commodity' is the combination of products, packing material, and containers that determines commodity classification. Commodities are assigned a Roman numeral character 'Class' based on the relative fire hazard. Class I is the lowest category and Class IV is the highest. The 'International Fire Code' includes a 'high hazard' category. The model fire codes and many insurance underwriters and research organizations have detailed lists of commodities and their classes based on live fire test results. Classifying a commodity can be a significant engineering challenge to ensure the appropriate level of fire protection is provided, and should not be taken lightly."
Coffee Break Training - Fire Protection Series No. FP-2009-34
United States Fire Administration: http://www.usfa.fema.gov/