Difference between deadline and finish no later than constraint

Dear Schedulers, According to my understanding, when a deadline is used in MS Project,then the late finish of the activity will be set to that deadline date. Also, when a finish no later than constraint is used, then again the late finish of the activity will be set to this constraint date. This means both of them do the same thing and the finish date can pass the deadline or constraint date which will generate negative total float. So I would be grateful if you tell me the real difference between these two because I find them the same. Bests Omid