A level players are the top tier players. They are players that would rate out as first division starters. Using scouting numbers, they are players that would be 55 or better. This is where the impact players come from. Most of these guys will be in the top 100, only a few will be left out.
B level players are players that I expect to have major league skills and could be impact level players if things break just right but are mostly just good useful major league players. Many of these guys won't make it but they have the skills that give them the chance to.
C level players are players that are likely to be second division starters, utility men, bench players, backups, 5th starters, bullpen arms and other less important role players. There may be a few that can exceed this. Many of these will never make an impact in the majors. Many will never make it at all.
d level players are those that should play in the majors this year but won't make a big impact. They are names to be familiar with if you are a big fan of your team but don't get too excited about them.
z level players are young. Under 20 for the most part. They are players I know enough about to know to mention them but not enough to place them in the B or C level. A handful will even be A level guys at some point, I'm sure, but they are a ways off.
This is a constant work in progress. I will update occasionally with rankings/rating changes. The age data isn't complete, so that will happen as I progress as well.
I plan to add other lists beyond team lists as well. Check back for information you won't find anywhere else on the internet.