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 newer version of Excel and uses functions not yet available in the version currently running.
In the example shown, the UNIQUE function is used to extract unique values from a range of data. UNIQUE is only available in Excel 365, and was introduced in 2020. 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.
