U.S. Foreign-Trade Zones: Background and Issues for Congress [Updated December 19, 2019] [open pdf - 2MB]
From the Summary: "U.S. foreign-trade zones (FTZs) are geographic areas declared to be outside the normal customs territory of the United States. This means that, for foreign merchandise entering FTZs and reexported as different products, customs procedures are streamlined and tariffs do not apply. For products intended for U.S. consumption, full customs procedures are applied and duties are payable when they exit the FTZ. [...] Current issues for Congress relating to the U.S. FTZ program may include the FTZ program in relation to the implementation of a series of tariff measures in 2018, as well as long-term issues, such as (1) whether U.S. FTZs encourage a misallocation of U.S. resources; (2) data availability issues; (3) security concerns; and (4) the U.S. employment and global competitiveness impact of FTZs. Broader considerations relating to the world zone network include (5) the effectiveness of trade zones worldwide as a tool for economic development; and (6) trade zones worldwide and worker rights."
CRS Report for Congress, R42686
Congressional Research Service: https://crsreports.congress.gov/