This Standard covers the formation and application of reference designations for electrical and electronics parts and equipment. The reference designations of this standard are intended for uniquely identifying and locating discrete items on diagrams and in a set, and for correlating items in a set, graphic symbols on diagrams, and items in parts lists, circuit descriptions, and instructions. This standard includes three methods for forming and applying reference designations; the unit number method, the location numbering method, and the location coding method. A complete reference designation may incorporate reference designations formed by the use of any of these methods at any level from basic parts to complete units. The unit numbering method has a long history of satisfactory use in all types of electrical and electronics equipment. The location numbering method and location coding method have been developed to permit rapid physical location of items in large, complicated equipments featuring multiple use of many identical, or closely similar, items. These methods shall be applied in such a way that duplicate complete reference designations do not occur in an equipment or system. Device function designations for power switchgear and industrial control use are not covered by this standard.