Google: Stick With ISO 3166-1 For Hreflang, Not 3166-2

Google responded to a webmaster asking if they can use ISO 3166-2 for their hreflang files saying you should probably stick with ISO 3166-1. Google's documentation specifically says they support ISO 3166-1 and makes no mention of ISO 3166-2.

Google writes in their documentation:

The value of the hreflang attribute identifies the language (in ISO 639-1 format) and optionally the region (in ISO 3166-1 Alpha 2 format) of an alternate URL.

There is no mention of ISO 3166-2, which is why Google is telling webmasters not to use it.

I'd stick to the documented ISO 3166-1 for hreflang. Do you have examples where -2 would be better to use?

— Google Webmasters (@googlewmc) October 24, 2017

Forum discussion at Twitter.