Any properly-formatted Google Spreadsheet can be used as the merge data source. When you select a spreadsheet file we will check the number of sheets within the spreadsheet: if there is more than one sheet we will prompt you to pick the specific sheet to use for the merge, otherwise we will automatically use the only available sheet. We will then scan the file and extract the first row of the selected sheet: the values in this row will be used as the merge data variables that can be merged into the template.