Flame resistant materials and clothing will not melt or drip when exposed directly to flames or extreme heat. In order to be classified as flame resistant a fabric must self-extinguish in 2 seconds or less after exposure to direct flame. The official test method to determine whether a fabric is flame resistant or not is ASTM D6413, aka the vertical flame test. Flame resistant fabrics gain their self-extinguishing properties either from the use of inherently flame resistant fibers or flame resistant treatments.
Flame retardant materials stop or slow the process of catching fire or reduce its intensity. These fabrics or clothing can be made from any material but must be treated with a special chemical to qualify as flame retardant. Fabrics used to make this clothing are not made with non-flammable fibers.