Service Based Duration is optional. You do not need to use it. Service Based Duration only works with the GAD booking screen. Service Based Duration will break some ABPro features - DO NOT USE unless you need to.
With 'Service Based Duration' you can create services, for each resource, that have duration or booking length defined.
You could define 30 minute timeslots but create 30 and 60 minutes services. When a customer selects the 60 minute service the booking will be made 60 minutes long. The 60 minute booking will overlay two 30 min slots.
Things to be aware of:
Once you start using service based duration, bookings are no longer constrained by timeslot boundaries. For example if you have timeslots at 11:00, and 11:30, then none at 12:00 (for lunch). Someone could make a 45 minute appointment starting at 11:30 and it will extend until 12:15. (See next point)
Bookings will extend into areas where there are no timeslots. The system will stop a booking going past the end of the day, or going into a book-off. You can define book-offs for lunch breaks and bookings will not encroach on them. The issue there is book-offs are date based so yo will need to define book-offs for every day. The create book-off screen allows you to create a series of book-offs between a start and end date so you can create a week of lunch breaks in one shot.
Features that depend on timeslot structure may not work correctly. For example 'Seats per Booking' depends on a fixed timeslot structure and will not operate correctly if you use service based duration.
ABPro cannot really go to midnight. It can only go to 23:59 as 24:00 is the same as 0:00. The use of service based duration with late night timeslots could run into the situation where a booking runs past midnight which will cause unpredictable display results in the GAD screen.
With Service based duration you cannot force gaps between bookings as you can with defined timeslots.