(Applies to: all versions of FusionBB)
A private topic is just like a topic posted publically to the forum, except it is private between two or more users. The private topic will show the users who are participating in the topic.
Users can leave the topic at any time. When a user leaves the private topic, there will be a post announcing that the user is no longer participating. Once a user is no longer participating, they cannot read the topic anymore.