When creating Excel files with Create, specifies which file format to use.
Starting with Excel 2007, Microsoft introduced a new file format for office documents. This new format relaxes some of the restrictions of the old file formats; for example a single worksheet can now have over a million rows instead of sixty-five thousand.
Excel files created with FileFormat.Xls can be opened in all versions of Excel. Excel files created with FileFormat.Xlsx can be opened in Excel 2007 and later, or Excel 2003 with the compatibility pack.