I added a formula to an existing spreadsheet using the SUMIFS and now when I save the file it says some features may be lost if I save it as xls. I've used the SUMIF elsewhere in the same spreadsheet forever and never got the message when saving.
I looked up xls vs xlsx and found that xls is compatible with older versions of Excel and is also faster. I didn't see anything in xlsx that is pushing me to save it as that, other than no longer getting the warning message.
I'm on a Mac.
Any opinions?