A subproject establishes a dependency of one Visual C++ project on another within a workspace file. You will want to create a subproject whenever you want to link two separate projects together.
For example, when a project builds an MFC executable program that depends on a static library. If the static library is a subproject of the project that builds the executable program, then the library will be updated before the executable program is built. Each configuration of a subproject is made a dependency of the corresponding configuration in the containing project. Building a configuration of the executable program also builds the same configuration of the subproject.