The master sheet is the heart of every merge: it decides what counts as a record. One row in the master sheet becomes one result in your output.
If your master sheet has 200 rows, a merge produces 200 records’ worth of output – 200 certificates, 200 catalog entries, whatever your layout and merge mode call for. Add a row, get another record. Remove one, and it’s gone from the output. The master sheet is your list.
Put the values that occur once per record here: a person’s name, a product’s price, an invoice number. Each column becomes a field you can map to a placeholder.
Anything that repeats many times per record – a product’s specs, an invoice’s line items – won’t fit in a single master row. That data goes on a secondary sheet and is pulled in with a data group.
In a single-sheet CSV, that sheet is automatically the master. In a multi-sheet Excel file, you choose which sheet is the master in the datasource settings (shown after you import the file); the others become secondary sheets.
When one row per record isn’t enough: Secondary Sheets.