Revelation Pets does offer a Waiting List. If you do not have availability, you can click the blue 'Add to Waiting List' button in the bottom left corner. The booking will be placed on a waiting list and you will see an alert if a run becomes free over the dates of the booking. Once the booking dates have passed, they will be deleted automatically from the waiting list.
There is also an option to have the waiting list button on the Online Booker if you want your clients to be able to add their booking requests to your waiting list if you don't have availability. If they do this, you will receive an email informing you a new booking has been added to the waiting list.