Throw and Catch Error Events

An error event attached to a task boundary can be set to catch any error, or errors thrown by the task to which it is attached.

For example:

when attached to a web service task, you can configure the catch error event to catch any WSDL fault associated with the web service operation that is invoked by the service task.
when attached to an email service task, you can configure the catch error event to catch emails when the email server is unavailable:

When attached to a call sub-process activity or an embedded sub-process task, the event can catch errors thrown by any of the following:

Activities within the sub-process.
End error events
Process interface error events (if the call sub-process activity references a process interface rather than a process)
Any other error throwing activity executed within the sub-process (including its sub-process tasks). This includes activities whose errors cannot be caught directly by attaching the error event (for example, throw message intermediate event and end event that invokes a one way message).