"Floor surfaces, because of their orientation and radiation feedback from heated upper layers of a fire, respond differently from walls or ceiling finishes. Consequently, they must be tested under different conditions. Floor covering samples are tested in accordance with American Society for Testing and Materials (ASTM) E648, 'Standard Test Method for Critical Radiant Flux of Floor-Covering Systems Using a Radiant Heat Source' or National Fire Protection Association (NFPA) 253, 'Standard Method of Test for Critical Radiant Flux of Floor Covering Systems Using a Radiant Heat Energy Source' that measure 'critical radiant flux': the amount of energy from an overhead test apparatus that will ignite the floor finish. The test is intended to simulate the conditions from a room flashover."
Coffee Break Training - Fire Protection Series No. FP-2012-38
United States Fire Administration: http://www.usfa.fema.gov/