Application Status Events
An application status event arises from changes in the processing status of an
application. For example, if a user brings an application to the foreground, the
Operating System sends a resume event to that application. Some of the work of
reactivating the application is done automatically, both by the Operating
System and by the Window Manager; the resume event is an indication for your application to take care of any further processing needed as a result of the
application being reactivated.