CSV (comma-separated values) is the simplest possible table: plain text, one row per line, cells separated by commas. No formatting, no formulas, no multiple sheets - just raw data. That simplicity is exactly why every system on earth can import it.
A .xlsx file stores formatting, formulas, charts and multiple sheets. Use it when humans will read or compute with the data, not just move it between systems.
Going the other direction strips everything except values - the active sheet becomes flat text. This is what most databases, accounting tools and import wizards expect.
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