Files with an "APPX" extension are basically an application package ready for distribution and installation. When uploading AppX packages for the Windows Store, developers must reserve the application's name, specify selling details and list advanced features (such as push capabilities). Once uploaded, the Microsoft Windows Store will run security and compliance tests on the AppX package before making it available for sale.
One of the principle challenges for application developers has been distribution of the application. Traditional computer applications have relied on clunky installers called wizards to sort out and set up the components of a program, but this paradigm does not work well across multiple mobile platforms.The AppX approach is particularly effective for distributing applications suited for multiple devices, including PCs, tablets and smartphones.