List of Countries using NSLocaleCountryCode

Apple uses the ISO-3166 standard.

"ISO standard ISO-3166" is accurate in most cases, but try selecting "Europe" as a region in iOS settings. You will get a return value of "150". Why "150"? Seems like a region code from here: http://en.wikipedia.org/wiki/UN_M.49. Or from here: http://site.icu-project.org/design/t/territory-region-apis

NSLocale gets its data from CFLocale which in turn gets its data from the ICU - International Components for Unicode (Apple keep copies here). The file /icuSources/common/uloc.cpp contains almost all the information we usually see returned.

However, /cldrFiles/supplementalData.xml may be the primary source. This file comes from the CLDR - Unicode Common Locale Data Repository.