The Type I material is intended for use in bulletin board covers, index guide cards and containers. Type II material is intended for use in ordinary glazing applications, transparent housings for instruments and safety goggles and helmets. Type III material is intended for applications requiring superior optical properties and resistance to outdoor weathering. Type III material is not for use as a glazing material on any aircraft. Reference to this specification on drawings or in product specifications may be made in order to specify flat or straight material to be used in the fabrication of a part. It must be clearly understood that the requirements apply to the flat or straight material prior to forming or fabrication and not necessarily to the material in its final form. The chief characteristics of cellulose acetate plastic sheets are their thermoplasticity, toughness, high impact strength and ease of fabrication. Their limited resistance to cold flow and the dimensional changes to be expected in service must be considered. Cellulose acetate plastics are only suitable for applications involving relatively low loads and in which some shrinkage on aging can be tolerated.