We have a single Confluence system split across multiple labels - all of which are individual companies within our parent.
However we are completely separate, and do not view each other's work. We have categories set up to allow spaces to be grouped under each separate company. People who work for Company A cannot see the spaces or even the category for Company B, and vice versa.
However, I can still @ mention users from Company B in a Company A article.
It would be very useful to us to be able to limit the list of users available in the @ mentions to just those who belong to the specific category.