In real time monitoring Eventsare received via Notifications when subscribing to an EventNotifier. The EventFilterprovides the filtering and content selection of Event Subscriptions. If an Event Notificationconforms to the filter defined by the whereparameter of the EventFilter, then the Notificationis sent to the Client.
In historical Eventretrieval the EventFilterrepresents the filtering and content selection used to describe what parameters of Eventsare available in history. These may or may not include all of the parameters of the real-time Event, i.e. not all fields available when the Eventwas generated may have been stored in history.
The HistoricalEventFiltermay change over time so a Clientmay specify any field for any EventTypein the EventFilter. If a field is not stored in the historical collection then the field is set to null when it is referenced in the selectClauseor the whereClause.