Duration is how long the booking occupies availability. A 60-minute booking starting at 10:00 normally uses capacity until 11:00.
Start interval or frequency controls how often a new start time is offered. With a 15-minute interval, customers may see 10:00, 10:15, 10:30, and 10:45 when capacity allows.
The interval does not shorten the service duration. Several bookings can overlap only when the configured capacity supports them.
If services take different amounts of time, configure duration per service. After changing timing settings, test the first and last slots of the day and a period with overlapping bookings.