This is the list of countries that may be selected in various places, for example in a user's profile. If blank (the default) the list in countries.php in the standard English language pack is used. That is the list from ISO 3166-1. Otherwise, you can specify a comma-separated list of codes, for example 'GB,FR,ES'. If you add new, non-standard codes here, you will need to add them to countries.php in 'en' and your language pack.