With the upcoming release of the 2007 Microsoft Office System, much attention has been focused on the new Open XML file formats. But there is also another new file format for Excel spreadsheets that you'll want to take a look at if you're building unusually large or complex spreadsheets: the the new XLSB binary format. Like Open XML, it's a full-fidelity file format that can store anything you can create in Excel, but the XLSB format is optimized for performance in ways that aren't possible with a pure XML format. The Excel team blog has some information about the various formats supported by Excel 2007.
The XLSB format (also sometimes referred to as BIFF12, as in "binary file format for Office 12") uses the same Open Packaging Convention used by the Open XML formats and XPS. So it's basically a ZIP container, and you can open it with any ZIP tool to see what's inside. But instead of .XML parts within the package, you'll find .BIN parts as shown to the right.
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