Hey there!
With this feature, you can set if you want the board to be accessible in all sites or just a specific site.
If you’re asking whether contributors can create boards and share those boards with other sites, that’s a great question (and this might be where your GDPR concerns are). Perhaps @yaaraa can weigh in.
If I misunderstood, please share more information with us about your concerns and we’ll be happy to get an answer for you 