Some clubs permit wait lists for reservations. When you create your reservation, if it overlaps with an existing reservation and wait listing is permitted you will be notified when you select your court or amenity that you will be on the wait list. Your reservation will also indicate it is a wait list reservation on the website and mobile app dashboards. Finally, your confirmation email will also indicate it is on the wait list.
If your reservation is upgraded due to cancellations you will receive an email with that information. You can set how much advance notice you need to take an upgrade from the website under My Profile->Preferences. The default is 24 hours. If you don't require that much notice you can reduce that amount and set it to zero if you don't require any advance notice.