In advance, you can reserve up to 3 FastPass+ experiences per day at one theme park. After you use all of the initial FastPass+ experiences on the day of your visit (or your arrival windows have passed), you can select one additional FastPass+ experience to use that same day (based on availability). The additional FastPass+ experience is a rolling FastPass+ experience—each time you use an additional FastPass+ experience, you can select a new one.
Additional FastPass+ selections must be made at a FastPass+ kiosk within the same park as the FastPass+ experience that you wish to select, and they can be made at a different park from your previous selections.