A wiring diagram shows how circuits connect electrically. It uses symbols, shows which pin goes to which pin, and ignores physical size, length and position. A harness drawing shows the physical object: the layout of branches, their lengths and tolerances, connector views, wire sizes, protective sleeving, clip positions and labels.
The wiring diagram tells the designer what must be connected. The harness drawing tells the factory what to build and the installer where it must go. A manufacturer cannot build from a wiring diagram alone, because lengths, branch positions and materials are missing. A complete enquiry therefore includes the harness drawing, the from-to wire list and the bill of materials. The data a quote needs is listed in the harness RFQ checklist, and APAR builds to customer drawings on the automotive and EV harness page.