From the Document: "Congress passed the U.S. Foreign-Trade Zone Act (P.L. 73- 397, 19 U.S.C. §§81[a-u]) in 1934, which authorized the creation of the FTZ [Foreign-Trade Zone] Program. The intent of the program was to encourage international trade and increase U.S. exports. FTZs are designated areas of the United States into which zone users may import goods duty-free, subject to certain requirements, for warehousing or production purposes within the zones. Duties are collected when goods leave the zones for consumption in the United States and not when exported. Similar programs exist in many other countries."
CRS In Focus, IF11348
Congressional Research Service: https://crsreports.congress.gov/