The determination of the value of an imported product, the "valuation", plays a significant role for the operation of a tariff system, and its harmonization to some extent is necessary for the prevention of protectionism. Duties on imported goods can be imposed in three different ways: they can be specific duties, that is, an amount of money imposed per unit of imports; they can be assessed on an ad valorem basis, which means a rate that is a "percentage of value"; or they can be a combination of the two. Art. VII provides a very broad roadmap for measuring actual value for purposes of customs valuation. Much of that uniformity is now provided by the Agreement on Implementation of Article VII of the General Agreement on Tariffs and Trade (GATT) 1994, which elaborates on Art. VII.
Available at: http://works.bepress.com/elizabeth_trujillo/11/