Files with the .XLSB extension contain spreadsheets, namely pages of rows and cells distributed in a table, containing data, formulae, charts, macros, and their assigned formatting.

Files of this type are unique to Microsoft Excel, versions 2007 and up, and coded binarily (BIFF12). They cannot be compressed and can be massive in size, but their reading and saving can potentially be faster. Other programs can read such files, but tend to save them in an Open XML format – XMLS.