HasNotifier and HasEventSource References are used to expose the hierarchical organization of Event notifying Objects and ConditionSources. An Event notifying Object represents typically an area of Operator responsibility. The definition of such an area configuration is outside the scope of this standard. If areas are available, they shall be linked together and with the included ConditionSources using the HasNotifier and the HasEventSource Reference Types. The Server Object shall be the root of this hierarchy.
Figure 24 shows such a hierarchy. Note that HasNotifier is a sub-type of HasEventSource. I.e. the target Node of a HasNotifier Reference (an Event notifying Object) may also be a ConditionSource. The HasEventSource Reference is used if the target Node is a ConditionSource but cannot be used as Event notifier. See OPC 10000-3 for the formal definition of these Reference Types.