Project subtypes let you customize or flavor the behavior of the project systems of Visual Studio. Customizations include saving additional data in the project file, adding or filtering items in the Add New Item dialog box, controlling how assemblies are debugged and deployed, and extending the project Property Pages dialog box. VSPackages implement project subtypes using COM aggregation.
The Visual C++ project system does not support project subtypes. Visual Studio itself uses project subtypes to implement SQL Server and Smart Device projects.