18 F is 18 gauge wire, F= Dark Green. The 18-F is shown on the official 55-59 GMC manual schematic. H.W. is not.
What schematic are you looking at?
If it is a aftermarket harness, assume it is harness company nomenclature. Call them if you need to know. Maybe if it runs across the motor, they want it "Heat Wrapped". Any wire near exhaust or motor, should be protected by raising it up, reroute or loom. PVC goes to 125 F and GXL goes to 250 F.
