Customs duty is mainly imposed on imported and selected exported goods specified by the Law on Customs Tariff. Customs duty is levied in accordance with the Harmonized Commodity Description and Coding System or Harmonized System. Most tariffs are ad valorem, which is a duty laid upon goods at a certain rate of their value. In certain cases, however, both ad valorem and a specific rate (e.g. a rate charged on a unit of goods) are given, and the tariff that gives the most revenue will apply. In general, the invoice price is the basis for computation of duty and normally applied to cost, insurance and freight (CIF) value for import and free on board (FOB) for export.