Question

What is meant by 'mapping the fields' in mail merge ?

Answer 1

Mapped fields were introduced in Word 2002. I believe they were intended to  

allow people to develop mail merge main documents that always used the same  field names, even when the document was connected to different data sources  that used different field names.

For example, suppose you have two lists of addresses in Excel, and they have  

the same columns, but with slightly different names, e.g. one uses

"Firstname" and "Lastname"