You're welcome
ISREF is a worksheet function which can be used in worksheet
=ISREF(Sheet1!A1)
It returns True if Sheet1 exists .. and if you change "Sheet1" to non-existing sheet, a window of "Update Values" will popup as an evidence that the sheet is not existing so this returns True or False
And using Not reverse the boolean value ..
so if the sheet doesn't exist so this part would return 'False' and Not False =True then the lines after the condition line will be executed
Regards