Events can be categorised by creating new EventTypes which are subtypes of existing EventTypes but do not extend an existing type. They are used only to identify an event as being of the new EventType. For example, the EventType DeviceFailureEventType could be subtyped into TransmitterFailureEventType and ComputerFailureEventType. These new subtypes would not add new Properties or change the semantic inherited from the DeviceFailureEventType other than purely for categorization of the Events.
Event sources can also be organised into groups by using the Event ReferenceTypes described in 7.16 and 7.18. For example, a Server may define Objects in the AddressSpace representing Events related to physical devices, or Event areas of a plant or functionality contained in the Server. Event References would be used to indicate which Event sources represent physical devices and which ones represent some Server-based functionality. In addition, References can be used to group the physical devices or Server-based functionality into hierarchical Event areas. In some cases, an Event source may be categorised as being both a device and a Server function. In this case, two relationships would be established. Refer to the description of the Event ReferenceTypes for additional examples.
Clients can select a category or categories of Events by defining content filters that include terms specifying the EventType of the Event or a grouping of Event sources. The two mechanisms allow for a single Event to be categorised in multiple manners. A client could obtain all Events related to a physical device or all failures of a particular device.