A foreign trade zone is physically inside the United States but treated as outside customs territory for the purpose of duty. That single legal fiction produces most of the reasons companies use them.
Duty is triggered by entry, not arrival
Ordinarily duty becomes payable when goods enter United States commerce. Inside a zone, goods can be admitted, stored and handled without that event occurring.
Duty is owed only when merchandise leaves the zone for the domestic market, and it is calculated at that moment.
Goods that leave the zone for export are never entered at all, so no duty is paid on them even though they physically sat on American soil.
Deferral has a cash value
A company holding inventory for months pays duty at the point of sale rather than at the point of import, which frees working capital in the interim.
For high-value or slow-moving inventory that timing difference is meaningful, particularly where the alternative is financing the duty payment.
Zones also allow weekly rather than per-shipment entry filing, which reduces the merchandise processing fees a high-volume importer would otherwise pay on every entry.
Manufacturing can change the tariff rate
Where a zone is used for production, components enter and a different finished product leaves, and the two may carry different duty rates.
With authorization, the company can choose to pay the rate applicable to the finished good rather than to the parts, which matters when components are taxed more heavily.
This inverted tariff situation arises in industries where raw inputs face higher rates than assembled products, and it is a principal reason manufacturers seek zone status.
Scrap and defects fall out of the calculation
Material that is consumed, scrapped or destroyed inside a zone never enters commerce, so no duty attaches to it.
A manufacturer with meaningful yield loss avoids paying duty on inputs that were never going to become sellable product.
Damaged goods can also be destroyed or exported from the zone rather than entered, which removes the duty and the disposal complication together.
The arrangement is supervised, not free
Zones operate under grants administered by a federal board, with sites activated individually and subject to customs oversight.
Operators maintain detailed inventory control systems that account for every unit admitted, transformed and removed, since the whole structure depends on that record.
Setup and compliance carry real cost, which is why zones are used by companies with sustained volume rather than by occasional importers.
Zones also sit inside state and local tax rules, and inventory held in one is often treated differently for property tax purposes. That treatment varies by state and is decided separately from the federal grant.