Finish No Later Than (FNLT) is a start/finish option that schedules a task to complete before the date you specify. If you select this value, then you must input a date in the planned completion date field.
This constraint is normally used when a project's schedule mode is set to Schedule From Start Date. When this is the case, it allows a project manager to provide a soft constraint on a task before it forces other dependent tasks to show as At Risk.
If FNLT is used on a Schedule From Completion Date project, then the constraint will schedule the task as it would an As Late As Possible task.