Those are the ISO 2 alpha codes. Most of them are the same ones as the IANA codes (i.e. internet tld's) but there are a few exceptions.
For example, the IANA code for the UK is UK (i.e. .co.uk) whereas the ISO 2 alpha code is GB. There others kinds of codes as well, with their own exceptions. In some kinds, bulgaria and turkey have issues.
Here's a list of several kinds of codes (though not all)
Country Codes