Alpha-2 code – a two-letter code that represents a country name, recommended as the general purpose code
Alpha-3 code – a three-letter code that represents a country name, which is usually more closely related to the country name
Alpha-4 code – a four-letter code that represents a country name that is no longer in use. The structure depends on the reason why the country name was removed from ISO 3166-1 and added to ISO 3166-3.
Country names might be removed from ISO 3166-1 for various reasons
Unassigned codes – codes that have not been assigned to country names.
Reserved codes – codes that do not represent a country name in the standard but that have been reserved for a certain use. While a code is reserved it will not be used in the standard to represent a country. There are different types of reserved codes.
User-assigned codes - If users need code elements to represent country names not included in ISO 3166-1, the series of letters AA, QM to QZ, XA to XZ, and ZZ, and the series AAA to AAZ, QMA to QZZ, XAA to XZZ, and ZZA to ZZZ respectively, and the series of numbers 900 to 999 are available.
NOTE: Please be advised that the above series of codes are not universal, those code elements are not compatible between different entities.
Formerly used codes – Codes that used to be part of the standard but that are no longer in use. See alpha-4 codes above.
Subdivision codes – code that represents the name of a principal subdivision (e.g province or state) of countries coded in ISO 3166-1. This code is based on the two-letter code element from ISO 3166-1 followed by a separator and up to three alphanumeric characters. The characters after the separator cannot be used on their own to denote a subdivision, they must be preceded by the alpha-2 country code.
For example – ID-RI is the Riau province of Indonesia and NG-RI is the Rivers province in Nigeria.
The codes denoting the subdivision are usually obtained from national sources and stem from coding systems already in place in the country.