CLS is a subset of CTS and defines a set of basic language features and programming constructs that all .NET applications have in common and the rules to which they must conform.
This enables interoperability between two .NET-compliant languages.
The rules defined in CLS serves as guidelines for third-party compilers designers, and library builders.
The languages supported by CLS can use each other's class libraries similar to their own.
APIs which are designed to be used as an Interface to access an application from a program by following the rules defined in CLS, can be used by all .NET compliant languages