The .APPXSYM file extension was introduced by Microsoft with Windows 8. The format is a script and source code one used by developers when creating Windows 8 (formerly known as metro style) applications and uploading them to Windows Store.
APPXSYM files are created when the developer packages all the application data in an .APPXUPLOAD file. Including .APPXSYM files in application packages isn't a mandatory thing to do but it is highly recommended since they contain all the symbols needed for app debugging and for creating app crash reports.
For submitting an application to the Windows Store without including the symbol packages, it is enough to create and upload only the .APPX file. However, .APPX files are limited to a maximum of 2 GB. Both .APPXSYM and .APPX files are part of the .APPXUPLOAD package.
How to create and open .APPXSYM files
The software used for opening and creating .APPXSYM files is Microsoft Visual Studio 2012. The process is a simple one and it is ran automatically during the creation of the .APPXUPLOAD application packages.
To create an .APPXSYM file users need to select the "Include Public Symbol Files, if any, to enable crash analysis for the app" check box available during the app package creation wizard.