CSV is a file format for Spreadsheet documents, and simpler and more widely supported than Microsoft Excel file format.
The CSV files used by phpList are formatted using tab characters. This allows you to use commas and semicolons in your subscriber attributes. For example, in subscriber address data.
If you open a CSV file downloaded from phpList (for example, a spreadsheet of list subscribers), your spreadsheet application may ask you which data delimiter to use when reading the data. If so, choose ‘Tab’.
Some versions of Microsoft Excel load CSV files with the wrong delimiter by default, in which case all data will all appear in a single spreadsheet column, and you will not be prompted to chose an alternative delimiter. You can change the delimiter used and select the correct one even after the file has been opened in Excel 2016, Excel 2013, Excel 2010, and Excel 2007. Follow the simple steps described in this Microsoft support article to do so (look for section “Change the delimiter that is used in a text file”): https://support.office.com/en-gb/article/Import-or-export-text-txt-or-csv-files