Mail Merge uses "MF_Fields" to allow you to personalize messages when you use a particular account to send/create them. The best practice is to fill in/create the MF Fields on the account prior to using these fields in your template and running a SAM campaign that includes that template.
If you do not add the data to the account BEFORE running your SAM campaign, you will be prompted for it when you run your campaign. If you enter data when prompted, you may end up with "hidden" data in those fields that is not viewable in Mail Merge. If this happens, you can edit this file: %Appdata%\Northwoods Software\MailMerge\Account.xml in Notepad to change or remove that data.
For more information on how to use these MF fields, refer to your Mail Merge manual - search for "MF_fields".