The documentation we pointed you to contains the basic information regarding fields, merge fields in particular and the mail merge action. If there is anything unclear in it, please let me know which part is misleading and I will make sure it is updated accordingly.
Like I previously explained, fields are special type of document elements and there are several ways to include one in a document. Importing or exporting a document that contains fields is possible to the formats that support the concept - DOCX, RTF and XAML. As to replacing a field with its resulting value, this is done when executing the mail merge action.
To your question, exporting a document containing fields to any of the formats mentioned above is performed just like exporting any other document. Based on the information in your last post I presume you are exporting to DOCX which results in the field being updated in their Result mode (please note, updated in Result mode
but not replaced
with their result).
If you try saving the document from the demo you pointed to, you'd notice only one record is available in the exported document, whereas performing mail merge results in all "merged" records in the same document. For future reference if you want to check whether something is a field (either when showing the document in RadRichTextBox or other editors like MS Word), you can simply right-click on it - in the context of fields, an "Update Field" item should appear in the context menu.
Additionally, if you do not want the fields to be exported in Result mode, but one of their other modes you can utilize the export settings available with DocxFormatProvider. More information on the approach is available in the Import/Export Settings
I hope this helps you achieve the desired result.
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