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You received a spreadsheet. It contains numbers that matter to someone, formatted in ways that matter to someone else. It's an XLSX — Excel's default since 2007, and the format that powers everything from household budgets to Fortune 500 financial models.
Like DOCX, an XLSX file is secretly a ZIP archive of XML files — rename one to .zip and look inside if you're curious. The older .xls format was a binary blob limited to 65,536 rows; XLSX handles over a million. It supports formulas, charts, pivot tables, conditional formatting, multiple sheets, and macros (though macro-enabled files use the .xlsm extension). The XML structure means other tools can read and write the format reliably, which is why Google Sheets and LibreOffice Calc handle most XLSX files without drama.
Excel (subscription) is the definitive editor. Google Sheets is free and handles 90% of typical spreadsheets. LibreOffice Calc is free and capable, though complex macros may need adjustment. For raw data exchange without any formatting concerns, CSV is simpler. For anything involving calculations, presentation, or that one colleague's 47-tab budget tracker, XLSX is the standard.