X-Coarse is best for beveling your edges and for repairing after the edge hits a rock. 

Coarse is one of the main ones you should own for beveling and repairing edges.

Medium is the second must-have file for well-performing edges as it gives you a good general finish for most skiing. 

Fine is an extra step for a great finishing polish which extends the life of your filing.

X-Fine is the finest file for the finest of margins; it gives a mirror like finish for the fastest, sharpest edges.