Sometimes, you might offer a service that is, for example, 1 hour long, and you only want your customers to schedule it on the hour (at 9:00, 10:00, 11:00, but not at 9:30). To do this, select your service, go into Service Details, and be sure that its duration is set to 1 hour (or however long you want your service to be).
Then, go into that service’s Appointment Rules and select Disable Personnel Selection. This will enable the ability to choose a Time Selection Mode, which you can change from Ask client’s availability to either Display time slots selection (for open booking) or Display time slots selection with automatic dispatch (for closed booking). Then, just below this, you will have the option to configure this service’s specific Appointment Interval. You can set this to 1 hour (or whatever you set your appointment duration to be).
Once this is done, you will see that you have only specific time slots available for your customers to book! You can do the same thing for 30 minute appointments, 45 minute appointments, and just about any other time interval you would want!