Each Event is of a specific EventType. A Server may support many types. This part defines the BaseEventType that all other EventTypes derive from. It is expected that other companion specifications will define additional EventTypes deriving from the base types defined in this part.
The EventTypes supported by a Server are exposed in the AddressSpace of a Server. EventTypes are represented as ObjectTypes in the AddressSpace and do not have a special NodeClass associated to them. OPC 10000-5 defines how a Server exposes the EventTypes in detail.
EventTypes defined in this document are specified as abstract and therefore never instantiated in the AddressSpace. Event occurrences of those EventTypes are only exposed via a Subscription. EventTypes exist in the AddressSpace to allow Clients to discover the EventType. This information is used by a client when establishing and working with Event Subscriptions. EventTypes defined by other parts of this series of standards or companion specifications as well as Server specific EventTypes may be defined as not abstract and therefore instances of those EventTypes may be visible in the AddressSpace although Events of those EventTypes are also accessible via the Event Notification mechanisms.