The grading system for bearings. In the middle of each wheel are two disks filled with ball bearings, one disk on each side of the wheel. The are separated through the wheel by a hub. The wheel mounting bolt passes through this hub. The surface of the ball bearings must be perfectly spherical to a fraction of a thousandth of an inch.
Smoother bearings cost more to make.
In my opinion, Killer Bees are fine, but not great. Cleaning is not worth the work, and they're never as good as new. A good blader does however own bearing cleaner and lubricant. The wheel diameter in millimeters, and the hardness rating. Different wheel brands and formulations feel different even at the same durometer rating.