Mapped fields were introduced in Word 2002. I believe they were intended to
allow people to develop mailmerge 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"

and the other uses

"Firstname" and "Surname"

When you use the Insert Merge Field dialog, typically the "Database fields"
radio button is selected and you see the field names that are in the Excel
sheet. However, if you click the "Address fields" button, you see a standard
list of field names including "First name" and "Last name". Suppose you now
click "Match fields" When you are connected to the first sheet you can match
"First name" to "Firstname" and "Last name" to "Lastname". If you are
connected to the other sheet, you can instead match "First name" to
"Firstname" and "Last name" to "Surname." Then, when you insert the field,
the \m switch is used to show that it is an "Address" or "mapped" field,
rather than a "Database" field.

When Word connects to a data source, it tries to recognise address field
names and do the matching (or is it mapping?) itself, so using Address
fields is made a bit easier for you. If you are used to using
matching/mapping, it will probably work reasonably well for you. If not,
personally I think sticking to the Database fields is a bit clearer.

Peter Jamieson