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They add a unique identifier to EFIVARS. That's why if you install a MS store application without being logged in, completely wipe/replace your disks you can still see the list of previous installed applications in the store.

I don't know about use of the store in general, but if you use the MS store to buy a Windows license they automatically convert your account and link the license to that account.

Source: built a new PC a few weeks ago and naively assumed they'd just give me a product key or ask how I wanted to receive it.

Don't know why you are downvoted, they do exactly that. Probably since a decade. They connect license and hardware ids.

Wait until you hear about the combination of hardware identifiers they use to validate that your license is only being used on one computer.