[file format] XAP is the file format used to distribute and install application software and middleware onto Microsoft`s Windows Phone 7/8 operating system, and is the file format for Silverlight applications. Beginning with Windows Phone 8.1, XAP will be replaced by APPX as the file format used to install apps on the Windows Phone platform...
Found on http://en.wikipedia.org/wiki/XAP_(file_format)
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