For improved backward compatibility, Microsoft Excel 97 has a new save file type option: Microsoft Excel 97 & 5.0/95 Workbook. When you save a workbook using this file type, Microsoft Excel writes two complete book streams. The first stream in the file is the Microsoft Excel 5.0/95 format (BIFF5/BIFF7), and the second one is the Microsoft Excel 97 format (BIFF8). The DSF record, which appears only in the BIFF8 stream, indicates that the file is a double stream file.
To distinguish the two streams, the BIFF5/BIFF7 stream is called Book, and the BIFF8 stream is called Workbook.