ISO standard 3166-1 gives codes for countries, dependent territories, and special areas of geographic interest.

The ISO 3166-1 alpha-2 code is a two-letter code. In most cases, this two-letter code is used as the top-level domain for that country on the Internet. The only exception is that the United Kingdom uses ".uk" instead of its ISO 3166-1 alpha-2 code of GB. The ISO 3166-1 alpha-2 code is also used as the first two letters of each currency code, including GBP for UK pounds sterling.

The ISO 3166-1 alpha-3 code is a three-letter code.

The ISO 3166-1 numberic code is a number associated with each country or territory in the standard.

TH is the ISO 3166-2 code for Thailand.

Yes, country code top-level domains are assigned based on ISO-3166 country codes, which are protected globally. (Except that the United Kingdom uses .uk instead of its ISO-3166 code of .gb; however, the ISO has agreed not to assign UK as an ISO-3166 code.)