When using subscriptions within Book That In, you have the ability to set a maximum capacity on each subscription.
If you set a number for the capacity, then Book That In will stop new people from booking as soon as you hit capacity for that subscription. Instead of giving them the option to book, the system will give your prospects the option to join your waiting list.
Once people are on your waiting list, you can view their details, remove people from the waiting list, and notify them by email when a place comes available.
This video explains how subscription waiting lists work in Book That in and how you can use them: