Exporting a workbook is more ambiguous than it sounds
A CSV holds one table. A workbook usually holds several, plus notes, plus a sheet someone left behind. "Convert this to CSV" therefore has no single right answer, and a tool that silently picks the first sheet will eventually hand you the wrong one. Here you choose, and if you need all of them you get one file per sheet in a zip.
Encoding is the setting that decides whether it opens
UTF-8 with a byte order mark is the right default: the mark is what makes Excel open the file with accents intact instead of guessing. But plenty of older systems — accounting packages, bank importers, legacy ERPs — reject UTF-8 outright and require Windows-1252. That is why both are offered. Characters that do not exist in Windows-1252 become question marks, which is stated rather than hidden.
Semicolon or comma, and why it matters
If the destination is a spreadsheet in a country that uses the comma as decimal separator, a comma-delimited file will split 1,50 into two columns. Semicolon is the safe default there. If the destination is a program expecting the international convention, pick comma. Tab is useful when the data itself contains both.
What does not survive
Formulas become their last calculated value, since a CSV has no way to express a formula. Formatting, colours, merged cells and column widths are gone: CSV is a text format with no notion of appearance. Cells that begin with =, +, - or @ are prefixed with an apostrophe, because otherwise the destination spreadsheet would treat them as formulas to execute.
Why it runs in your browser
Workbooks are where organisations keep the things they would least like to leak: payroll, pricing, client lists. Uploading one to convert it is a disproportionate exchange for a format change.
How dates come out
Excel does not store a date; it stores a number counting days from the end of 1899, and the date you see is that number wearing a format. A CSV has no formats, so the number has to become text, and the choice of text matters. Dates are written as 2026-03-14, and with a time as 2026-03-14 09:30. That is the ISO order, chosen because it is the one spelling no system reads backwards: it cannot be mistaken for the American or the European convention, and it sorts correctly even as plain text.