To add to the above:
A classification is an indicator of the best result you've had for the last four years. Classifications are awarded based oh where you finish and are weighted according to 1) how many people were in the tournament and 2) how tough the tournament was.
The intent of the classifications is to allow better "seeding" of fencers when making up the pools for a tournament, so the best don't face each other early and eliminate each other while allowing a lesser talent from another pool to advance.
They are only a rough measure of your ability--actually, an indicator of your single best day in the last four years--and classifications can be influenced by many things other than raw talent, not least of which is just plain dumb good (or bad) luck--hence the constant quarreling on this board and elswhere over whether some "ratings" are "real" or not.
Don't look at them as "ranks", like the belts in a martial art. They aren't. I've known C's who could beat B's and A's, and have known at least one U who beat C's and D's regularly but didn't have a classification 'cause he never went to tournaments.