Google’s Matt Cutts explains the thinking behind how Google decides if a country code can be used globally or whether it’s only going to be targetted to a specific country.
The basic thinking is that if the country code is in active use in the region then you won’t be able to target it to anywhere else.
Bear in mind, of course, that the geo-targetting is only one factor that impacts a site’s SEO.
At the moment Google considers the following domain name extensions to be “global”:
Generic Top Level Domains (gTLDs)
Regional top-level domains
Although these domains are associated with a geographical region, they are generally treated as generic top-level domains (much like .com or .org).
Generic Country Code Top Level Domains
Google treats some ccTLDs (such as .tv, .me, etc.) as gTLDs, as we’ve found that users and webmasters frequently see these more generic than country-targeted. Here is a list of those ccTLDs (note that this list may change over time).
Full details here