Chapter 73 is the "Articles of Iron or Steel" chapter in the Harmonized System, covering various finished or further-worked articles of iron or steel formed by casting, forging, rolling, drawing, extrusion, etc., as well as iron or steel structures, containers, pipe fittings, cables, etc. This chapter does not include iron or steel raw materials (Chapter 72), machinery (Chapter 84), or electrical machinery and equipment (Chapter 85). The core logic is "articles made primarily of iron or steel materials, classified by material properties rather than by use or function," and only iron or steel articles meeting specific structural characteristics remain in this chapter. Heading 7309 specifically refers to "reservoirs, tanks, vats and similar containers for any material (other than compressed or liquefied gas), of iron or steel, of a capacity exceeding 300 liters, whether or not lined or heat-insulated, but not fitted with mechanical or thermal equipment." Its scope covers large iron or steel containers for holding liquid, gas, or solid materials, such as oil storage tanks, water tanks, fermentation tanks, silos, reactor shells, etc. The key qualifications are a capacity exceeding 300 liters and no mechanical or thermal equipment; otherwise, they may be classified under Chapter 84 or Chapter 85. The first 2 digits "73" = Chapter 73 "Articles of Iron or Steel," defining the material category as finished iron or steel products, excluding iron or steel raw materials and mechanical/electrical equipment; digits 3-4 "09" = heading 7309, indicating "reservoirs, tanks, vats and similar containers for any material, of iron or steel, of a capacity exceeding 300 liters," which is the qualification on container use and capacity; digits 5-6 "00" = the subheading under this heading, indicating no further subdivision by material, use, or structure, and all large iron or steel containers meeting the description of 7309 are classified under this subheading. Therefore, 730900 is a complete code of "chapter + heading + general subheading," and classification focuses on determining whether it belongs to Chapter 73 iron or steel articles, whether it meets the capacity >300 liters requirement, and whether it has no mechanical or thermal equipment. The goods are steel storage tanks, made of iron or steel, belonging to Chapter 73 articles of iron or steel; the capacity typically exceeds 300 liters, meeting the heading 7309 "capacity exceeding 300 liters" qualification; and they are not fitted with mechanical stirring, heating, or refrigeration equipment, so they do not constitute machinery of Chapter 84 or Chapter 85. Therefore, they are classified under 730900, rather than 7308 iron or steel structures, 7310 small containers (capacity ≤300 liters), or 8419 heat exchange equipment.