Analysis
HTS 2604.00.00 covers nickel ores and concentrates under heading 2604 (nickel ores and concentrates). US Customs collects import statistics for it under 2 ten-digit suffixes, reported in Ni kg.
For statistics it divides in two: 2604.00.00.40 for nickel content and 2604.00.00.80 for other metal content. Both pay the same duty; the suffix records which kind was imported.
The normal trade relations rate here is zero, shared by about 97% of the 39 tariff lines in chapter 26, a useful check when a neighbouring dutiable line is a candidate. Free here means free of the base duty only: origin-specific Chapter 99 duties such as Section 301 can still be owed.
It is the only US line under subheading 2604.00, so the six-digit code used by every Harmonized System country and the eight-digit US line describe the same goods; a foreign tariff or an exporter's invoice showing 2604.00 points straight here.
Census statistics map these goods to NAICS 212230 (Copper, Nickel, Lead, and Zinc Mining), an industry of 61 US establishments employing 16,844 people. That concordance is how imports of this line are compared with US output.