initial link to dropbox (basic question)

Hi there, I searched forum/internet, couldn’t find the answer.
When I install Updraftplus on my website and I choose backup to dropbox – how does the plugin know to which dropbox to link?
This is not the situation where I want to re-authenticate, but simply: the first time.
I have 2 dropbox accounts, both seemed active at my laptop when I choose ‘dropbox’ as backup location.

It’s not based on the exact e-mailadress, because I use a different mailadress for my site then for the dropbox account.
Although the have the same domain-name. Is that where Updraftplus looks at?

Thanks you for looking into this question!