You are able to keep a separate list of appointments on the Waitlist when you don't have availability on your active schedules.
When making the appointment you can click on the Add to waitlist box before hitting OK to save the appointment.
Click on Schedule, Waitlist. There are tabs for the modules in your database.
You can also add appointments to the waitlist by click/drag or clicking the Add button.
Whenever you check a pet out early or cancel/delete an appointment on the schedules, you will get a popup to remind you there are pets on the waitlist so you can move them if needed.
To move the appointment from the Waitlist onto the schedule, open the appointment and uncheck the Add to waitlist box.