Quoting William Weber via db-wg <db-wg@ripe.net>:

It is not really as the Netherlands and the EU recognise Kosovo as independent country.
By your logic Taiwan/TW should also be not selectable, it however is - and Taiwan is recognised by much less countries ("UN members") than Kosovo.
 

 

Two letters country codes are ruled by ISO 3166-1 standard.

And, according to ISO 3166x, X* codes are NOT  "country codes".

XK is just a so called "user assigned code", temporary used by just few organizations (European Commission and SWIFT), but it's NOT an official country code.
Other examples of user assigned codes:
XI, used by UK Government for Northern Ireland,
XU, used by World Intellectual Property Organization for "International Union for the Protection of New Varietes of Plants"
XZ, used for installations in international waters 

Looking at EU, the XK code is "a designation without prejudice to positions on status, and is in line with UNSCR 1244/1999 and the ICJ Opinion on the Kosovo declaration of independence. (with asterisk and footnote in written documents, only the first time that Kosovo is mentioned)."

https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Glossary:Country_codes#cite_note-1

Regards

 

Franco Tauceri
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