While configuring a new forum and specifying the access-modifications, the following options are open to you:

Level of access (who will be allowed to access the forum)
• anonymous
• registered users only
• moderators/administrators only

type of access (how is the forum displayed, how can it be accessed)
• public
• private
• public (invisible)

Build 99 has the following rules for the accessibility of your forums:
The password-field is invisible now - instead you can rely on OPN's automated password-administration. You won't have to type your password any more.
Type 'public forum' is visible for all users.
Type 'private forum' needs configuration to enable a user to enter.
Type 'public' (invisible) is visible only for the webmaster. If the webmaster publishes the URL, a user can enter the forum.