The XLS format dates from Excel 2003. It was limited to no more than 65,000 rows. XLSX format solved this with the release of Excel 2007; it goes out to a little over 1,000,000 rows.
I’ve often been asked what the difference is between XLS and XLSX, and when I tell people that this is the main difference, most people shrug.
After all, how often do you need to know this?
Apparently, someone at Public Health England didn’t know this and managed to lose 16,000 coronavirus cases in its “track and trace” system.
See below and pause to admire this screw up:
https://www.theverge.com/2020/10/5/21502141/uk-missing-coronavirus-cases-excel-spreadsheet-error