Bookings are reserved in the connected Office 365 calendar in a reserved state. A booking is only considered confirmed when the form has been completed. This setting specifies how long, in minutes, a booking will be reserved for before being cancelled and removed from the calendar.
A background task runs every 10 minutes that will cancel and remove reserved events that were created greater than the specified number of minutes in the Expiry field. As such when setting this setting you should choose a suitable expiry time that allows your customers enough time to complete their form. For example 60 minutes should be enough time to complete most forms.
When an event is reserved by one customer it cannot be booked by another customer.