Iain Sproat wrote:

That's a useful property, but it gives (a) wrong answer in the case of Taiwan. There may be some spiritual sense in which "Taiwan" and the "PRC" are really part of a greater China, and that might be the case someday (it seems that the KMT and CCP no longer have a fundamental dispute about the political system in 'China'.)

Practically, however, when it comes to visas, passports, economic statistics, and just about everything else, "tw" is a different entity than "cn".

When I made my list of countries I let ISO-3166 be the primary authority and then did a few hand edits. You'll still find a number of strange and disputed cases in there. For instance, "hk" is officially listed as an administrative division of China. "ps" is also a strange case. On the other hand, "yu" has been withdrawn, so there is some effort to keep things up to date,

Ultimately you're not going to find one list of countries that is universally agreed upon because there will always be some places on the margin that are disputed. Most of them, however, aren't as practically important as "tw".