HTS heading 7309

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 — 3 codes under this heading, with the duty on each.

2 of these lines state no rate of their own and inherit one from the line above, which is how the schedule is read. Each says which line it came from.

Source: Harmonized Tariff Schedule of the United States, U.S. International Trade Commission (public domain). Classification and the duties that finally apply to a shipment are a licensed customs broker's call.