First, within your event module navigate to Registration Website > Ticketing & Form > Manage ticketing and form set up. Here, you can select the ticket type you would like to select the bookable sessions for by clicking on the ticket name (e.g. below):
Under the "Edit Ticket Class" pop up which will open when you click on your ticket, you must enable session booking and uncheck "show all sessions" to ensure that the ticket owner cannot book all sessions. Once this has been done you can go ahead and select the sessions you wish that ticket holder to be able to book:
Remember to "Save" (top right hand corner) once done!