CLS which stands for Common Language Specification is a subset of CTS. So, CTS can be defined as a collection of sets of rules and specifications that a language that runs under the .NET framework has to meet, to be CLS-compliant.
Whereas CTS can be defined as Common Type Systems represents all the classes and structures that are related to Data Types and is used by every other language which runs under the .NET framework. And CTS is a superset to CLS.
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