Both Windows and Windows Phone support launching applications based on file extensions or via a custom uri protocol. This is the equivalent of “open in” capability that other platforms have and it is a fundamental mechanism for sharing data from one application to another. For example you might have a dropbox application which can list various files but doesn’t provide a mechanism for opening or editing all possible file types. Instead when a user clicks on a file to open it, the application can simply attempt to launch the file, allowing the platform to pass off the file to the appropriate application. This is implemented by taking a copy of the file and passing it to the destination app. Unfortunately this then means that the view/editing application can’t save it back to dropbox. The round-about way to save the changes is to then launch the file, this time with the file being picked up by dropbox.
What’s interesting is that this is expressly prohibited by the Windows Store App certification requirements. Point 3.1 states that applications can’t implement bi-directional data exchange:
My view is that this clause was written by a completely different team (probably a security “expert”) to the one that actually implemented file/uri handling and doesn’t really understand why they were implemented in the first place. In Windows you can implement Share contracts but this is not really a great way to data exchange between apps – it works well for “share to social” or similar scenarios. Take the Facebook Login for Windows Phone this uses bi-directional data flow to authenticate users via Facebook for applications wanting to connect to Facebook (similar to the way iOS works for Facebook auth). With the current Windows Store requirements I don’t see this as being possible.