If you open up a spreadsheet that you did not create, you may run into the "__xlfn" prefix before certain functions. The generic meaning of xlfn is "unsupported function". Typically, it indicates that the worksheet was created in a more recent version of Excel than the one currently running, and uses functions that version does not have.
In the example shown, the UNIQUE function is used to extract unique values from a range of data. UNIQUE is available in Excel 2021+ and Excel 365. When the workbook is opened in an older version of Excel, the __xlfn prefix appears. Note that the original result is still displayed. However, the formula will not update to show a new result if data changes.
Solution or workaround
If you open the same workbook in a version of Excel that contains the missing function(s), the _xlfn prefix will disappear and the function will calculate normally. If this is not possible, you might be able to use another formula that does not rely on the newer function.
