All US imports are subject to duty unless exempt. US customs territory includes the 50 states and Puerto Rico. Dutiability equals classification plus valuation. Duty paid is a function of duty rate times value times quantity. The duty rate is determined by classification.
Valuation is determined by value methodology, and quantity is determined by physical measurement.
The scheme that the US uses for customs classification is used internationally, and based on the Harmonized System. Classification takes place by headings and notes. There are incomplete and unassembled articles, classified with the complete article. The use provisions determine the customs classification. The rates of duty includes two columns.