document file is the document you are merging to - the data file is a
document that contains the data to be mail merged.
i.e.: the document file contains the form letter and mail merge fields. The
data file holds the data needed to complete the mail merge fields so you can
send the form letter.
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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours