To define regions, the SDIA uses the NUTS classification —  the Nomenclature of Territorial Units for Statistics — developed by the European Union.

The classification is defined by Regulation (EC) No 1059/2003 of the European Parliament and the Council.

The NUTS system is structured into three main levels:

• NUTS 1 - Major socio-economic regions:

  These are the largest territorial units, typically representing major regions or areas with populations between 3 and 7 million inhabitants.

• NUTS 2 - Basic regions for the application of regional policies:

  These intermediate level regions usually have populations between 800,000 and 3 million people.

• NUTS 3 - Small regions for specific diagnoses:

  These are the smallest territorial units, typically with populations between 150,000 and 800,000 people.