This SAE Recommended Practice defines a recommended practice for implementing circuit identification for the circuits of the power and signal distribution systems of the Class 8 trucks and tractors. This document provides a description of a supplemental circuit identifier that shall be utilized in conjunction with the original equipment manufacturer’s primary circuit identification as used in wire harnesses but does not include electrical or electronic devices which have pigtails. The supplemental circuit identifier is cross-referenced to a specified subsystem of the power and signal distribution system identified in Section 5.