If you're publishing the form for internal use (in other words, with an
Exchange SErver, not being sent over the Internet) you should not check this
box. Checking the form definition box sends the whole design of the form
with the form. It is useful when sending custom forms over the Internet.
However, in Outlook 2002 and 2003, it also prevents code from behind the
form from running. Bottom line, don't use it unless you really need it.