Customs areas

A customs area is an area designated for storage of commercial goods that have not yet cleared customs. It is surrounded by a customs border. Most international airports and harbours have designated customs areas, sometimes covering the whole facility and including extensive storage warehouses.

For the purpose of customs duties, goods within the customs area are treated as being outside the country. This allows easy transshipment to a third country without customs authorities being involved. For this reason, customs areas are usually carefully controlled and fenced to prevent smuggling.

However, the area is still territorially part of the country, so the goods within the area are subject to other local laws (for example drug laws and biosecurity regulations), and thus may be searched, impounded or turned back.

The term is also sometimes used to define an area (usually composed of several countries) which form a customs union, a customs territory, or to describe the area at airports and ports where travellers are checked through customs.

...
Wikipedia