Iron scrap is primarily categorized by its composition, thickness, and cleanliness. Buyers should distinguish between HMS 1 (Heavy Melting Steel), which consists of recyclable steel at least 1/4 inch thick, and HMS 2, which is thinner (1/8 inch). For high-quality smelting, prioritize 'Bonus Grade' or 'Plate and Structural' (P&S) scrap, as these offer higher density and lower impurity levels. Always verify the chemical composition, specifically looking for low levels of copper, tin, and phosphorus, which can embrittle the final steel product.