The event is signaled from lib/exe/ajax.php if the AJAX call is not recognized and allows you to add your own functionality to the AJAX backend.
$data is a scalar containing the AJAX call name in $_POST['call'].
Remember to use preventDefault() (and possibly stopPropagation()) when handling an unknown AJAX event. This keeps other handlers from corrupting your returned data. Also use a unique AJAX call name to avoid collisions
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.