With custom scheduling, you can choose when to observe your target. This is useful when what your are observing is time critical, such as in the case of stellar occultations or exoplanet transit, or when you want to be in full control of the schedule.
Note that one of the disadvantages of this mode compared to Automated scheduling is that if your request fails for some reason, such as unsafe weather, it will not be automatically rescheduled. If you then reschedule your observation, you will jump to the back of the queue as you will need to submit a new request. By using automated scheduler, you can maintain your priority when observations fail.
In custom scheduling you will need to use the Scheduling calendar to book your session. Find out more about how to use the scheduling calendar.