A customs union is an agreement between countries under which customs duties on trade between them are abolished and a common customs tariff is applied to countries outside the agreement.
In practice this means that goods brought into one country belonging to the customs union can then be sold freely in the other countries of the group, without additional border charges. The rules of the customs union are of key importance for international transport. They facilitate trade, strengthen economic cooperation and help create a more cohesive market between the member states.
In practice these rules apply, among others, to trade between the European Union and Turkey, where the A.TR certificate is used.