Chapter 73 covers articles of iron or steel, including seamless pipes, welded pipes, pipe fittings, wire ropes, chains, nails, bolts, springs, etc. This chapter has a broad scope, ranging from pig iron and ferroalloys to various finished steel products, but excludes ferrous waste and scrap (Chapter 72) and certain specialized machinery (Chapter 84). Anti-skid chains, as a type of iron or steel chain, are a typical commodity of this chapter. Heading 7315 covers chains and parts thereof, of iron or steel, including articulated link chains, anti-skid chains, other chains and parts. Anti-skid chains specifically refer to anti-skid devices used on vehicle tires or shoe soles, usually composed of chain links, connecting pieces, and fasteners. This heading includes not only finished chains but also parts of chains, but attention must be paid to distinguishing them from tools of Chapter 82 or vehicle parts of Chapter 87. Code 731520 is a six-digit subheading. The first 2 digits '73' represent Chapter 73, articles of iron or steel; digits 3-4 '15' represent heading 7315, i.e., chains and parts thereof, of iron or steel; digits 5-6 '20' represent subheading 731520, specifically anti-skid chains. In HS classification, the subheading level further refines the scope of goods, and anti-skid chains as an independent subheading are distinguished from other chains (e.g., 731511 for roller chains, 731512 for other articulated link chains, 731519 for other parts of chains). Anti-skid chains are made of steel, function to increase friction and prevent slipping, and belong to chain-type products. Their structure is usually connected chain links, conforming to the definition of 'chains' under heading 7315. They are not classified under other subheadings of 7315 because of their specific use; nor under Chapter 87 vehicle parts, because anti-skid chains are independent accessories, not inherent vehicle components; nor under Chapter 82 tools, because they are not hand tools.