Isso pode ser melhor explicado pela seguinte descrição.
A multi-targeting pack, or MT pack, is a set of reference assemblies that corresponds to a particular .NET Framework platform and version. A reference assembly is a .NET Framework assembly that typically has no method bodies and no internal or private APIs. Reference assemblies contain just the information a compiler needs.
For example, there are multi-targeting packs for the .NET Framework 3.5, the .NET Framework 4, Update 4.0.x for the .NET Framework 4, the .NET Framework 4.5, Windows Phone 7.5, Windows Metro style apps, Portable Class Library, and so on.
Visual Studio installs a set of multi-targeting packs, and so do SDKs such as the Visual Studio SDK, the Silverlight SDK, and the Windows Phone SDK. Typically, multi-targeting packs are installed under
“%ProgramFiles(x86)%\Reference Assemblies\Microsoft\Framework”or“%ProgramFiles%\Reference Assemblies\Microsoft\Framework”on a 32-bit computer).
A seguinte imagem da Microsoft fornece clareza à descrição acima.
DoIneedthesepackagesifIonlywanttouseVisualStudiotomakeWP10Mobileapplications?
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