The registration rules allow you to decide which participants are able to choose which sessions during the registration process. The registration rules can be apply only by defining participation types.
They can be defined in Configuration - Registration Settings - Registration Rules.
Here, you will have the defined participation types and the list of the sessions visible in registration. Tick the sessions to which a certain participant type is able to attend. If the sessions are not selectable, it means that you haven't added them to registration. In order to do so, go to the Agenda, choose the session, click on Edit and tick Add to registration.