The cell is the basic unit of configuration and administration in DCE. In terms of security, a cell is the set of principals that share a secret key with an instance of the registry service. Therefore, each instance of a security server (together with its replicas) defines a separate cell.
From the perspective of security only, a cell is sometimes also known as a realm or security domain. (The term realm is often used in Kerberos documentation, and so may be more familiar to some readers than is the term cell.) A security cell is always configured to coincide with a corresponding CDS cell, and perhaps Distributed File System (DFS) cell as well. DCE documentation always refers to such a collective configuration of services as a cell.