I'm sure there are, though I can't think of any off the top of my head. The concept for clusters though is to try and avoid single pixels or disconnected lines of pixels by keeping things chunky and grouped.
To illustrate, here's a "bad" cluster and a "good" cluster:

"Bad", there's a lot of hatching and visual noise that creates a grainy, cluttered look.

"Good", there's no longer any single-pixelwidth tangents or free-floating pixels, resulting in a smoother yet still textured appearance.
Obviously these aren't applicable, and in some instances noisier textures are desirable, but hopefully that helps with understanding the concept!