To protect wiring against abrasion, a plastic braid may be applied.   

One of the braiding machines that weaves poly threads around harnesses.  

While a standard black is normally used, an identifying filament may be added.     

Cable Braiding

In this scene one may see a partially finished harness being wrapped with braid with the remaining unbraided wires underneath the braiding machine.  

The harness progresses  through the machine from the bottom.