A deadline is a target date indicating when you want a task to be completed. Imposing an inflexible constraint such as Must Finish On, that sets the finish date can lock up your schedule in ways you don't want. Instead, if you just need a reminder, you can use the deadline field of a task. This will create a green downward arrow on the Gantt chart, that marks the deadline date. The advantage of deadlines is that they keep track of its finish date without locking the schedule.