Understanding CLR, CIL, CTS, CLI, BCL, FCL represents a meaning

 CLR: the common language runtime (Common Language Runtime)

 CI L: Common Intermediate Language (Common Intermediate Language, referred CIL)

 CTS: Common Type System (Common Type System)

 CL I: Common Language Infrastructure (Common Language Infrastructure)

 BCL: Base Class Library (Base Class Library)

 FCL: Framework Class Library (Framework Class Library)