Messages in comments

["Joel Jacobs" <jj-at-netexp.net>]

Hello,
>From the http://www.linuxcnc.org/handbook/RS274NGC_3/RS274NGC_33a.html
RS274/NGC specification I find the following:

A comment contains a message if "MSG," appears after the left parenthesis
and before any other printing characters. Variants of "MSG," which include
white space and lower case characters are allowed. The rest of the
characters before the right parenthesis are considered to be a message.
Messages should be displayed on the message display device.

What is the message display device?  I was thinking it would pop up a
message window but that doesn't seem to be the case.  How's this used?

Thanks..
Joel