How data sources work in a MS- Word mail merge

You can think of a data source as a table. Each column in the data
source corresponds to a category of information, or data field — for
example, first name, last name, street address, and postal code.

The name of each data field is listed in the first row of cells, which
is called the header record. Each subsequent row contains one data
record, which is a complete set of related information — for example,
the name and address of a single recipient.

When you complete the merge, individual recipient information is
mapped to the fields you included in your main document.