Waiting lists can be enabled on an event by event basis by selecting the the “enable waiting list” option on the embed event / ticketing widget management page.
If the waiting list box is selected, then when tickets sell out, the “book now” button will become a “join waiting list" button and allow customers to register to be notified if new tickets become available.
You can keep track of the waiting list for an event via the “Manage Attendees” section.
If tickets do become available, e.g. through a cancellation or new tickets going on sale, the waiting list will be emailed automatically, notifying them that new tickets are available on a first come, first serve basis.