Quick answer
A cable assembly is usually a single cable, or a small group of cables, cut to length and fitted with connectors or terminals at the ends. A wiring harness is a larger, organised set of wires, terminals, connectors and protective sleeves that is laid out to a drawing and routes power and signals to many points in a vehicle or machine.
A harness often branches, with several breakouts feeding different components, and may include conduit, grommets and strain relief. APAR builds harnesses to customer drawings for automotive, EV and rail use, as described on the automotive and EV harness page and the railway coach harness page. Simple earthing leads and crimped socket assemblies sit at the cable assembly end of that range.