I found the related article here --

http://help.cbp.gov/cgi-bin/customs.cfg/php/enduser/std_adp.php?p_faqid=572&p_created=1048279878&p_sid=HzLqhWij&p_accessibility=0&p_redirect=&p_lva=572&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9MjkzLDI5MyZwX3Byb2RzPTAmcF9jYXRzPTAmcF9wdj0mcF9jdj0mcF9zZWFyY2hfdHlwZT1hbnN3ZXJzLnNlYXJjaF9ubCZwX3BhZ2U9MSZwX3NlYXJjaF90ZXh0PWxhd2Z1bCBwZXJtYW5lbnQgcmVzaWRlbnRz&p_li=&p_topview=1