The PolicyApplication class is used as a convenient point for defining relationships to different managed entities. It has four principal subclasses: PolicyServer, PolicyDecisionPoint (PDP), PolicyExecutionPoint (PXP) and PolicyEnforcementPoint (PEP). <br/>Figure below has been simplified by removing all relationships except the ContainsPolicyApps aggregation. The OCL expression shown in the ContainsPolicyApps aggregation ensures that a PolicyServer has at least one or more PolicyDecisionPoints, PolicyExecutionPoints, and PolicyEnforcementPoints. This is the minimum amount of functionality required to specify a PolicyServer. Note that a PolicyServer can have additional functionality specified; however, that is only relevant in the system view, and hence will not be discussed further in this document.<br/>