Explanation
Fulfillment preferences are customizable entities that let you choose how schedule assistant results are displayed, like with neat hourly appointments or morning and afternoon time windows.
By default, the schedule assistant displays results based entirely on resource schedules and the earliest available time, such as 10:39 AM. With fulfillment preferences set to hourly, the same resource's availability shows as 11:00 AM. This makes it simpler for the scheduler to view and understand availability and communicate it to the customer.
Intervals
Intervals display schedule assistant results in neat time slots that dictate start time of subsequent bookings.
When configured as 30-minute intervals, the schedule assistant will display a resource available at 9:27 AM as available at 9:30 AM and will book the start time (arrival time) for 9:30 AM. This includes travel time for onsite requirements and work orders, meaning travel time will begin before 9:30 AM, and a field resource will arrive and start work at 9:30 AM.
Reference:
https://docs.microsoft.com/en-us/dynamics365/field-service/set-up-time-groups