SO and Internet country codes
The columns A2, A-3, and NUM give the international country codes as per ISO 3166 and its later amendments.
The INET column is supposed to be the Internet top-level (country) domain code. It seems to have been copied verbatim from column A2, which may not be correct. For instance, it gave GB for Great Britain, whereas most Internet addresses there seem to use UK. So, treat this colum with suspicion...