They basically determine how many moves deep Stockfish 8 looks ahead to figure out the rating for each move. There can be a significant difference between the minimum and the maximum but basically the bad moves and good moves will be called close to the same in each one.
But even the max. at chess.com doesn't go that deep. If I set Stockfish 8 to look 20 ply deep on my fairly fast computer (3.2 GHz Quad-Core Processor) using Lucas Chess, it takes about 70 seconds per half move to analyze. I'm guessing the deepest on chess.com is between 12-14. In my last game, just 14 move game, a move chess.com labeled an inaccuracy was not so with 20-ply Stockfish and a move barely considered an inaccuracy by chess.com was considered a very poor choice by 20-ply Stockfish.
I was just wondering because the minimum scan is enough for me. What do the other 2 scans have to offer?