Okay people. Can you please explain this tier system? Last year when we went on our annual Disney vacation in September, FP+ was just being tested and we were not part of it. Obviously with the full roll out, we will be using it this November when we return. My understanding was that you get to preselect your Fast passes for each day--3 a day. Now I am seeing something about tiering? What?

Seems like the old days of A-E tickets.
Can you please educate me as to what all these tiers mean?
Also, if you use your 3 FP's up, do you get to get others like in days past?
Any help would be very much appreciated!