HTS heading 7305

Other tubes and pipes (for example, welded, riveted or similarly closed), having circular cross sections, the external diameter of which exceeds 406.4 mm, of iron or steel — 33 codes under this heading, with the duty on each.

8 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.