I dunno whats the official definition, I just remember the old (phpbb) forums having a complex roles structure and privileges so a subset of community would often have access to subforums or threads that other people can’t see. You’d have a public face of the forum and then private categories within it that wouldn’t be visible from outside.
This is more of a reddit-like news bulletin where everything is public and open by default - in the case of fediverse even more so since everything automatically gets pushed to other servers that you have no control over.