US Customs requires importers to hold an import bond to clear goods into the US.

Used by an Importer of Merchandise to guarantee that the proper amount of duty is paid, and that the imported goods are in compliance. The bond is issued to U.S. Customs and Border Protection (CBP).  The bond amount is usually based on 10% of the duties, taxes and fees paid by the importer during the previous 12 months. The minimum bond amount is $50,000. This is by far the most common type of customs bond.  

Single Entry Bond is a one-time Importer Entry Bond for a particular import shipment, which can only be used for one Customs transaction. The bond amount for a single transaction bond is equal to the total entered value of the merchandise plus all duties, taxes, and fees. However, if the merchandise is subject to other government agency requirements or visa/quota requirements, the customs bond amount would be equal to three times of the total entered value. 

Customs requires a single ISF bond to support ISF Filing, if importer does not have a Continuous bond.