The other two questions that come up about multitargeting are:
How does Visual Studio ensure that the application is allowed to execute only against a specified SKU?
The answer lies in the additional app.config file added to .exe project templates, which specifies the profile and CLR runtime version. Visual Studio adds enforcement to prevent apps specified as NETFX4 Full in app.config to run on NETFX4 Client. If the app.config file is missing, the application will run only on the Client Profile.
For down-targeted projects, what compiler does Visual Studio use to compile my project?
For non-web applications, Visual Studio uses the 4.0 compiler to compile projects that target all the framework versions. The 4.0 compiler ...