Customs duty levy based on ad valorem valuation governs tariff classification and duty assessment under statutory valuation rules. The Customs Act establishes the levy of duties on imported and exported goods, with items and rates set in Tariff schedules and imports classified under the Harmonized System. Duties may be specific, ad valorem, or combined. The primary basis is ad valorem taxation determined by statutory valuation rules implementing the WTO Valuation Agreement for both imports and exports.