Most HTSUS headings describe goods by what they are. Use provisions describe goods by what they are used for, and they come in two kinds that work very differently. A principal use provision, governed by Additional U.S. Rule of Interpretation 1(a), turns on the principal use in the United States of the class or kind of goods to which the import belongs, at or immediately before the date of importation. The importer's own intended use does not decide it.
An actual use provision, governed by Additional U.S. Rule of Interpretation 1(b), is satisfied only if the use is intended at the time of importation, the goods are actually put to that use, and proof is furnished within three years after entry. Whether a heading is a use provision at all, and which kind, changes what evidence matters. For principal use, courts weigh factors such as physical characteristics, channels of trade, and the expectation of the ultimate purchaser. The terms here explain each test and when it applies.