The document discusses how the Model-View-ViewModel (MVVM) pattern helps share code between Universal Windows Platform (UWP) apps targeting Windows and Windows Phone. MVVM separates an app into platform-independent and platform-specific components, allowing views, view models, and models to be shared while platform-specific projects handle each platform. The document demonstrates how to use MVVM, input-output coupling, and shared XAML to build Universal Windows apps that converge Windows and Windows Phone.