When should the xlsm or xlsb formats be used?

Question

Since Excel 2007, Microsoft has split the classical .xls format to several formats (in particular, .xlsx, .xlsm, .xlsb).

Answer 1

They're all similar in that they're essentially zip files containing the actual file components. You can see the contents just by replacing the extension with .zip and opening them up. The difference with xlsb seems to be that the components are not XML-based but are in a binary format: supposedly this is beneficial when working with large files.

https://blogs.msdn.microsoft.com/dmahugh/2006/08/22/new-binary-file-format-for-spreadsheets/