From ply count to load range
Modern commercial tyres no longer use literal cotton plies, but the load range letter still maps to a historical ply-rating convention. The higher the letter, the more air pressure and load the casing is rated to carry.
Match the axle, not the truck
Specify load range to the heaviest axle the tyre will sit under, then confirm the load and speed index covers single and dual fitment at your operating speed. Under-specifying is a safety and warranty risk; over-specifying can add cost and affect ride quality.