The "hard" and "soft" part is actually the clearcoat. When someone refers to a hard clearcoat, it means the clearcoat is tougher to polish. Scratches are harder to polish out of a hard clearcoat because the polish needs to work harder or longer to grind away the necessary amount of clear to remove the scratches/swirls.
You also may need heavier polishes to work with a harder clearcoat.