Files with the XLSB extension are known as Microsoft Excel Binary Spreadsheets. Just like XLSX, the default Excel format, XLSB stores data in worksheets. Each worksheet has a grid of cells arranged in columns and rows. Cells can contain text and numbers of various fonts, colors and sizes. XLSB files may also include charts, pivot tables, graphs, mathematical functions and VBA (Visual Basic for Applications) projects. Additionally, XLSB files can also include macros.

The main difference between the two file types is that XLSX files are based on the Open XML format which stores all content in a compressed archive while XLSB files store spreadsheets in a binary format (BIFF12).

Because they’re not archived, XLSB files are usually larger but they’re also faster to parse and save/load. Their binary structure also enables them to perform more efficiently within Microsoft Excel. XLSB are compatible only with versions 2007 and above of Excel.