Hyperglide (HG) ®
A system of ramps and special-shaped teeth on Shimano rear sprockets that permits much smoother shifting than older systems. HG sprockets are designed so that, as the chain moves from one sprocket to the next, it will engage the new sprocket before it has completely derailed from the old one. This makes for smooth, silent shifting.
Hyperglide requires that the teeth of adjacent sprockets be oriented specifically with respect to one another. It also reduces interchangeability to some extent. For example, a 17-tooth sprocket that is designed to be used next to a 16 tooth sprocket will be shaped differently from one designed to be next to a 15 tooth.
That doesn't mean that you can't customize a Hyperglide cassette, but does mean that if you do customize, you will lose the Hyperglide functionality for some of the shifts.