Fixed: This means that the appointments will stack up according to the duration in minutes you choose for your appointments. So if the duration is 10 min and your day starts at 08:00 the front end will list the availability sequentially at 08:00 08:10, 08:20, 08:30 etc. per the duration you selected for that event. Flexible: This means that regardless of the duration of your appointment, the availability will always stack on the nearest interval of 15 minutes (by default). Thus your 10 min appointment will stack sequentially at 08:00 08:15, 08:30, 08:45 etc. And if you book that 10 min appointment, the next series will be 8:10, 8:25, 8:40, etc. The default interval can be changed in /application/controllers/Appointments.php at the variable:
$interval = $availabilities_type === AVAILABILITIES_TYPE_FIXED ? (int)$service_duration :