Ron, how is your wife?Yes, they are closed by invitation--very restricted. The content on them is not available without the permission of the owners of the page. It is not like BYC where content is searchable via Google.
An example is wanting to build a notwaste bucket feeder. A BYC thread shows up quickly for building one. If it were on a restricted facebook page, It would not be found.
Gotta love BYC!