This is an international agreement instrumental in harmonizing and simplifying customs procedures in the exchange of goods between the European Union, EFTA countries, and other countries such as Turkey. The Convention facilitates the movement of goods through a standardized customs transit procedure with a single transit declaration and a single customs guarantee.
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A21987A0813%2801%29