You can list all available regions by calling /v1/regions API.
It returns a list of supported regions and their id which you can use as a parameter for other APIs.

For example, if you want to get recent articles for California state, find California region id in /v1/regions response:
{ "id": 76, "name": "California", "short_name": "ca", "GeoName": "California", "HasSubRegions": false }

Then take this id = 76 and pass it to /v1/regions/articles like this:
/api/v1/regions/76/articles

If region id is not required and not set:
/api/v1/regions/articles
API will return world data