CAS numbers are used to identify specific chemical substances, and are valid for identifying chemical substances in CLP and on the Safety Data Sheet. CAS numbers are the main chemical identification used by the chemical industry worldwide.
CAS number (or CAS registry number), refers to the Chemicals Abstract Service number. This is run by the American Chemical Society, and its database has been developed since 1957, see https://www.cas.org/content/chemical-substances/faqs .
CAS numbers take the format of between 2 to 7 digits (hyphen) 2 digits (hyphen) 1 digit, or (XXXXX)XX-XX-X .
CAS numbers are supposed to be unique to specific substances, but there can be duplicate entries for the more obscure chemicals.
Another difficulty is that for some polymer-type substances, such as ethoxylates or propoxylates, a single CAS number is used to identify a whole range of substances based on the substance being ethoxylated, regardless of the number of moles of ethylene oxide present.
Despite these minor complaints, CAS numbers are the main numerical method of identifying chemicals in use worldwide.
CAS numbers are usually used alongside EC numbers for chemical identification within CLP, and can be used on their own in the absence of a valid EC number.
If you regularly need to find CAS numbers for your chemical structures, there is a paid-for subscription service, but there are other routes to find this information (e.g. the ECHA databases etc).