2 numbers for the metropolitain départements (except the two departements on Corsica which have 2A and 2B). The codes are identical with the standard departement numbering.
2 letters for the territorial collectivites and for the overseas territories.
The purpose of this family of standards is to establish a worldwide series of short abbreviations for places, for use on package labels, containers and such. Anywhere where a short alphanumeric code can serve to clearly indicate a location in a more convenient and less ambiguous form than the full place name.
