ISO 3166-1 country code
From Just Solve the File Format Problem
The ISO 3166-1 standard defines three types of codes to identify a country, or similar geographic division.
Two-letter country code
Examples of two-letter country codes are "US" for the United States, "UY" for Uruguay, and "UZ" for Uzbekistan.
One use of two-letter country codes is as top level domain names.
Three-letter country code
Examples are "USA" for the United States, "URY" for Uruguay, and "UZB" for Uzbekistan.
Numeric country code
Numeric codes have three digits, including leading zeroes.
Examples are 840 for the United States, 858 for Uruguay, and 860 for Uzbekistan.