ASTM E648 (the same test as NFPA 253) is ASTM's Standard Test Method for Critical Radiant Flux of Floor-Covering Systems Using a Radiant Heat Energy Source.
So it's a mouthful, but what does it measure?
This test measures the flame spread of flooring systems, like carpet, in corridors and exits. Radiant heat is applied by means of a gas fueled panel, inclined at a 30º angle over horizontally installed flooring mounted on concrete (just like real carpeted corridors). This test is designed to simulate thermal radiation levels from an adjacent fire, and to rate different materials for use in different occupancies.
This is important, because if there's a fire in the building, you don't want the means of egress to become a flaming inferno.
The radiant flux is measured in W/cm² and higher numbers are more resistant to spreading the fire.
Materials are rated as either Class I or Class II, with Class I being required in institutional occupancies like hospitals, nursing homes, jails and other facilities where it would take longer for occupants to get out. Class II would be used for other occupancies except residential. Whether or not a building is sprinklered can also determine what class of materials are required.