Here is the definition of the region subtag in the BCP 47 official documentation https://tools.ietf.org/html/bcp47 :
Region subtags are used to indicate linguistic variations associated with or appropriate to a specific country, territory, or region. Typically, a region subtag is used to indicate variations such as regional dialects or usage, or region-specific spelling conventions.
This part is very clear, but then the definition continues:
It can also be used to indicate that content is expressed in a way that is appropriate for use throughout a region, for instance, Spanish content tailored to be useful throughout Latin America.
The sentence that content is expressed in a way that is appropriate for use throughout a region can be interpreted in many different ways.
Is it still strictly related to the language itself or is it about the content rather than the language?
For instance, if my website has a dedicated subdomain for Greece gr.mywebsite.com with a content adapted to Greek users but the language used is American English, should I use a en-GR language tag? Or the content doesn't matter and the language tag should just be en-US?
Let's take as example English, there is the UK/GB version and the US version (along other). They are not about the content, but how grammar and spelling works.
So with en-GR you should use grammar and spelling of English as used in Greece. I expect you should be more similar to British English.
If you write American English, the language is American English, per definition.
Nobody will check the language tag to see the content target. The content itself and the people who read it would define it.
Warning: do no create random combinations. The locale should exists or you will get weird results, and results depending on browsers. Consider that browsers do not implement all possible combinations.
BTW you are looking on the wrong place. IETF explains the syntax. You should looking on how to apply on web pages, and how browser should interpret it, so you should check: Choosing a Language Tag (W3C) and related documents: language tags and Authoring HTML & CSS