The System Frameworks Projects layer is concerned with parts of the application that provide system service, system infrastructure functionality, or other shared functionality. This functionality might not be specific to any given application. Examples include an HTML page cache service or an object query cache (name service/load balancing service).
The Systems Frameworks Projects layer can be used by any layer within the architecture. In addition, strongly-typed DataSets reside in a System-type layer. When some pieces of shared functionality are used only within an application and other pieces of shared functionality are shared among several applications, consider separating them into different System-type projects.