The app window, or some element outside the ReaderView was the target of the event.
This value is only used if the ReaderView option constrainEngineEventsToReaderView is set to false.
A ReaderDocument was the target of the event. For example used when clicking publication content.
The ReaderView was the target of the event, but it was outside a ReaderDocument or ReaderViewAnnotation. For example used when clicking outside a reader document, but inside the ReaderView element.
A ReaderViewAnnotation was the target of the event.
Describes the framework component that was the target of an EngineEvent.