By choosing annual registration (fall and winter sessions), you are saving your spot for the entire year. That way, you don't have to register again in winter. Also, your activites are all on the same bill, which facilitates payment tracking.
Your registration for the fall session is already selected (see the price at the top of the page). If you select the option below, the winter session will be added to your cart. If you wish to register for the fall semester only, select "Do not register annually". Your spot for the winter session will not be guaranteed then.