Establishes a system of designation for polystyrene thermoplastic material, which may be used as the basis for specifications. Applicable to all amorphous polystyrene homopolymers. Applies to materials ready for normal use, unmodified or modified by colorants, additives, fillers etc. Does not apply to expanded polystyrene, styrene copolymers, homopolymers of substituted styrene and those modified with other polymers such as elastomers.