This question must have been asked many times, but I can't just find the right answer...

When searching on non-US google, such as http://www.google.com.tw/, what criteria are used by Google when user searches with the pages from option - the right most radio button option right under the search bar?

For example searching on google.com.tw with the rightmost option "Taiwan's websites", you'll get bunch of .com sites in the results some of them hosted from outside of Taiwan.

?

The most obvious is the TLD, since in most countries it is hard to get a local TLD without having some connection with the country. The strength of this might vary by country, since there are a limited amount of countries and Google is capable of tweaking manually the emphasis on this aspect.

A second criteria would be the IP address. Again, this factor might differ by location. It is very common for Canadian websites to be hosted in the USA, but much rarer for US websites to be hosted in Canada. So Canadian hosting is a pretty good filter for a Canadian website.

A visible mailing address in the country might be another key, if all files named contact or contactus are searched.

An above normal number of links from other sites that are considered to be from that country would be another clue, especially if Google can identify a few directories that will link only or primarily to local websites.

And when all else fails, never under-estimate the power of "because I said so!" :)