You cannot change any cells outside of the cell that the function was
called from, if the function was called from a worksheet cell.

You CAN change other cells inside of a function only if the function is
called from a VBA macro.

You can put the line...

Application.Caller

....inside a VBA routine to return information about how Visual Basic was
called (see "Caller Property" in Microsoft Excel Visual Basic Reference).

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Regards,
Bill Renaud