I'm not entirely sure what the region picker in search does. It doesn't limit my searches to the region's primary language or to content targeting that region, because even with my region set to Australia I still frequently get the American version of sites (e.g. PcPartPicker) or even other language content (frequently French personally). Does it just downrank those results? It seems to do so for some results e.g. pcpartpicker gives au.pcpartpicker.com as top result. But for other searches e.g. pcpartpicker ram the top result is still the American site, and the Australian site is second. I thought I could fix this via a rank adjustment, but lowering pcpartpicker.com also effects all subdomains...
I also can't find any documentation about the region picker.

- The search region selector should have documentation
- The search region selector should more reliably increase ranking of target region sites
I don't imagine this being an easy one to solve in the general case, but it would also be good to see duplicate content detection. For example, these two results are the same but target different regions specified via subdomain:
I don't really care at all about the American results, especially if I've set my region to Australia. I'm not sure if I want to filter those results completely, or just downrank them significantly. Up to you, I guess.