Sometimes several classifications may naturally be associated with each other, having many concepts in common. These classifications form a cluster. The clustering may be based on physical location.
In functional clustering, classifications are centered around functions. In data clustering, classifications are centered around data, while in object based clustering, classifications are centered around objects. The object based paradigm uses the concept of class hierarchies to naturally express clustering.
Encapsulation is a concept similar to the concept of clustering. In encapsulation, an enclosure is placed around a grouping and only threw well-defined openings are interactions allowed. A cluster may not have such a well defined enclosure.