Error events are used to model an exception in a business process. Errors are thrown by error end events and caught by error start events and error boundary events.
The errorRef property in the errorEventDefinition of an error event element will match against the id of an error when viewing the XML Editor.
Error events are displayed as a lightning bolt icon inside different shapes that differentiate between the event types. A solid lightning bolt represents a throwing event, whilst a hollow lightning bolt represents a catching event.
To create a new error use the + symbol against an error event such as a start error event, or make sure no BPMN element is selected by clicking on a blank section of the process canvas and the Edit Errors button will be visible in the right-hand properties panel.