by CBP and the U.S. International Trade Commission. This lengthy schedule provides rates of duty for each class of goods. Most goods are classified based on the nature of the goods, though some classifications are based on use. Duty rate Customs duty rates may be expressed as a percentage of value or dollars and cents per unit. Rates based on value vary from zero to 20% in the 2011 schedule. Rates may be based on relevant units for the particular type of goods (per ton, per kilogram, per square meter, etc.). Some duties are based in part on value and in part on quantity. Where goods subject to different rates of duty are commingled, the entire shipment may be taxed at the highest applicable duty rate. Procedures Imported goods are generally accompanied by a bill of lading or air waybill describing the goods. For purposes of customs duty assessment, they must also be accompanied by an invoice documenting the transaction value. The goods on the bill of lading and invoice are classified and duty is computed by the importer or CBP. The amount of this duty is payable immediately, and must be paid before the goods can be imported. Most assessments of goods are now done by the importer and documentation filed with CBP electronically. After duties have been paid, CBP approves the goods for import. They can then be removed from the port of entry, bonded warehouse, or Free-Trade Zone. After duty has been paid on particular goods, the importer can seek a refund of duties if the goods are exported without substantial modification. The process of claiming a refund is known as duty drawback. Penalties Certain civil penalties apply for failures to follow CBP rules and pay duty. Goods of persons subject to such penalties may be seized and sold by CBP. In addition, criminal penalties may apply for certain offenses. Criminal penalties may be as high as twice the value of the goods plus twenty years in jail. Foreign-Trade Zones Foreign-Trade Zones are secure areas physically in the United States but legally outside the customs territory of the United States. Such zones are generally near ports of entry. They may be within the warehouse of an importer. Such zones are limited in scope and operation based on approval of the Foreign-Trade Zones Board. Goods in a Foreign-Trade Zone are not considered imported to the United States until