There are two private tags.
Who are they visible to?:
A document tagged with "Board Only" is visible only to the directors and officers. Any managers such as Property Managers, CEOs, Exec Dir, Exec Assistants, etc will not see these documents in their documents page.
A document tagged with "Board & Managers Only" is visible to the directors, officers, and managers. The Recording Secretary is also able to view these documents.
This means that if either of these tags are used, owners and members can not see them.
When to use these tags?:
The usage of these tags is mostly for an association with a defined membership or a condo corporation with owners. The members or owners have access to BoardSpace as an "Other" (check out the Settings - Others) but you do not want them to have access to all the associations' documents so the private tags are added.
Also see Who is a manager?