When you add a class to a live course, you can save it as a draft or publish it. Learners cannot see a draft class or register for it. Learners can only see a class and register for it when it is published and added to a custom channel.
If you wish to use a published class as an unconfirmed session, you can use enrollment settings to allow users to self register and join the waitlist. Set the maximum registrants to one and register yourself for the course forcing all interested learners to join the waitlist. If enough learners join the waitlist you can register them for the class.