The Nomenclature or Continuous Register Population by Population Unit contains the detailed report of population groups existing at the 1st January in every municipality, including their codes and their populations relating to that date.

Information is obtained from the updates of population groups carried out by Town Councils in their municipalities and that, in compliance with current legislation, they communicate to the INE at least once a year. Populations are calculated using population figures from the Annual Municipal Register Revisions, which are officially declared by the Government at the proposal of the INE, with the favourable report of the Register Council, through a Royal Decree.