'Plate C' refers to a drawing or 'plate' which gives clearance dimemsions for equipment. If a car has 'plate -C' designation it means it is able to clear obstructions that fall outside of the listed dimensions. Conversly anything within these dimensions will foul the equipment. Tunnels, bridges, loading docks, all come to mind.
"I may not have a brain, but I have an idea!" BOB, benzoate ostylezene bicarbonate.