XLS is the legacy Microsoft Excel spreadsheet format used before Excel 2007. It relies on a binary file format (BIFF) to store data in worksheets, rows, and columns. Despite the prevalence of the newer XLSX format, XLS files are still used in many legacy financial systems and automated reporting tools. XLS sample files are used to test spreadsheet import/export features in business applications. Developers use them to verify that formulas, macros, and cell formatting from older files are preserved during migration or viewing. They are also crucial for testing libraries that specifically handle binary Excel parsing.