According to Google, these are the generic top-level domains (gTLDs) which do not specific a country. Google treats the following as gTLDs that can be geotargeted in Webmaster Tools.
Generic Top Level Domains (gTLDs)
ccTLD means Country Code top-level domains. Country code domains are created to be used by individual countries.
ccTLDs (e.g., example.de, example.fr)