The region code localized the page geographically at a region level and is used to generate the locale setting
Example: the french language is present in the region:
This regions even if they have the same language may have different formatting such as the thousand separator or decimal points.
The google search engine uses this information to localize your content.
You can set a region code:
The format supported is the ISO 3166 alpha-2 country code. The complete list of values can be found on Wikipedia on this page: list of ISO3166 country codes
The default value is set in the sitelanguageregion configuration.
If there is no value, the locale server country is retrieved (if the internationale extension is installed)
A page is by default localized to the region set in the siteLanguageRegion configuration