XLSB, Excel Binary Spreadsheet (.xlsb)
The XLSB Spreadsheet file format
A document with an XLSB file extension is a spreadsheet file that was originally created in Microsoft Excel. There is a lot of confusion around the various Microsoft spreadsheet files, but most of that confusion is simply related to how old the software is that created the spreadsheet. Earlier versions of Excel saved files in the XLS format. This was a Binary file format that was used through Excel 2003. The XLSB extension came into existence in Excel 2007. The XLSB file is more efficient for the system to open and run and it also gives people using it the ability store files that are much larger.
Technical details of XLSB files
The XLSB file extension is a type of spreadsheet created by the most recent version of Microsoft Excel. The previous versions of Excel should all have the ability to open XLSB files, however you will likely lose some of the additional functionality that was added since the previous versions of Excel. The XLSB file combines ZIP compression and the XML architecture. Microsoft has indicated that they will make the product “open” and available for anyone to use. However, the XLSB file extension is not the same as the Open Document standard. There is a separate XML spreadsheet standard being developed for the Open Document Standard.