CAS Numbers (often referred to as CAS RN or CAS Registry Numbers) are universally used to provide a unique, unmistakable identifier for chemical substances. They are permitted by American Chemical Society to be used to identify a chemical substance or molecular structure when there are many possible systematic, generic, proprietary or trivial names. CAS Numbers are used in many other public and private databases as well as chemical inventory listings and, of course, are included in all CAS-produced databases.

Each CAS Number identifier:

Is a unique numeric identifier
Designates only one substance
Has no chemical significance
Is a link to a wealth of information about a specific chemical substance

Source: https://www.cas.org/content/chemical-substances/faqs; http://www.rsc.org/.