Responding to a prompt with a null value ! will not necessarily cause the application to abort, but it can force a suitable default to be used, where this is the most-sensible action. A further use is when an optional file may be created, such as a lookup table; a ! entered in response to the prompt for the filename means that no file is to be output. Many tasks use null as a default for optional files. In some applications, a null ends an interactive loop.
Responding to a prompt with !! will abort the application. This
process includes the various tidying operations such as the unmapping
and closing of files. Any other method of stopping an application
prematurely can leave files mapped or corrupted.
KAPPA --- Kernel Application Package