How does the callback key?

I'm having difficulty understanding the inclusion of the callback key described here: http://docs.e-signlive.com/doku.php?id=esl:e-signlive_guide_event-notification private EventNotificationConfig eventNotificationConfig; public string URL = "http://my.url.com"; public string KEY = "abc"; // Register for event notification eslClient.EventNotificationService.Register(EventNotificationConfigBuilder.NewEventNotificationConfig(URL) .WithKey(KEY) .ForEvent(EVENT1) .ForEvent(EVENT2) Is the "KEY = 'abc'" the api key? or some password? It's not clear how this works, how it's validated? The example here appears to describe how to set a key is but not how to use it/validate it.