1. When Excel displays [Compatibility Mode] in the title bar, it indicates that you have opened an old XLS file, usually created by the 1997 through 2003 versions of Excel

2. Detecting the Biff version in an XLS file: A BoF record, identified by the record type byte (byte 1) with value of Hex 09, marks the beginning of a Book or Workbook stream in a Biff file