A cluster is a patch of perceptually connected pixels which are part of the description of a shape, volume or texture or any other element.
A cluster may be thought of as a fragment of any element, as a sample of anything, as seen by the eye or as imagined.
Shaping and shading a cluster gives direction and orientation to the cluster itself which also serves to give direction and orientation to the element which the cluster aims to help define.
Shadow and light on a cluster can be thought of as individual (sub)clusters as well. Single, perceptually separated pixels are not clusters by definition.