Hi Marc,
Since my recent post in Free Support, I have tested the applications "Forum", "Board" "Mixed" and "Category" and can confirm that only the latter two will completely hide sub-forums set to be viewable by members of certain groups.
There are two further restrictions for this privacy to work. The private sub-forums must be immediate child forums of the root forum and the add-on "Smart Application Pages" must be activated.
It only takes a couple of minutes to test this for yourself. Register a second user on your forum that you can use as a test user, adding and removing group memberships as required. Then change your forum to each of the application types logging on first as a user with the group membership that you want to see the private sub-forum and then as a member without that access.
I always recommend that any forum administrator should have a second registration to use as a test user for just such a purpose. It ensures you can write definitive responses to any user who is having problems with your forum and it allows you to double check that what you plan should happen is happening for those in the "Anyone" and "Registered" groups. This is especially important if you use the "Member" group, or create special ones for "Moderator" or other special classes of user on your forum.
I go further and have a specific test forum where I check how complex CSS rules will work and other particular customisations that people ask about in Free Support.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.