In a customs union, trade barriers are either reduced significantly or eliminated altogether. Title II of the Maastricht Treaty, Article 3a, stipulates that one of the primary activities of the European Union (EU) is to eliminate customs duties and restrictions on the import and export of goods between member states. However, there are some stipulations regarding the free trade of goods between EU member nations.

The intra-European Union trade statistics (INTRASTAT) declaration for arrivals and dispatches is a monthly declaration that provides EU authorities with statistics on foreign trade between European Union member states (for example, between Germany and France) when a monetary threshold is exceeded during the previous or current year.

Even though internal tariffs have been eliminated for trade within the EU, the authorities keep statistical records to assess international competitiveness, keep track of each region's dependency on arrivals and dispatches, and monitor price changes within EU borders.

Since trade barriers have been removed within the European Union, there are no restrictions on trade between EU member states. That means that domestic goods can be transferred between member states freely with only one stipulation – an INTRASTAT declaration must be filed with the responsible authorities for statistical purposes.

In some instances documentation is required for shipping goods between member states of the European Union. For example, the Single Administrative Document (SAD) is sometimes required for both external and internal trade. For an explanation of these documents, see the list below.

Documentation is required when dutiable goods enter the EU from an independent country and are not cleared through customs in the country of arrival. For example, such goods may be imported to an EU member state and then transferred in-bond to another EU member state. For such transfers, the use of a T1 document is necessary. Generally, customs processing then takes place in the country of final destination.