You can use that from either the Cities page (linked from the top left menu) or from a region’s page.
Each city shows it’s region/country info in the top of the left column. Those are links, so you can go from your city page to its region’s page & then search/show/hide cities in there.
That cities page does need some help. Only allowing search by city name is fine but it would be better if you could filter by country/region from that main list.
I’m not sure if there’s more than the one idea post for that: City search results