A database of chemical compounds, certain mixtures, and biological sequences maintained by the Chemical Abstracts Service of the American Chemical Society. In the registry every entry has a unique CAS registry number (CASRN). This has three parts: up to six digits, followed by two digits, followed by one digit. For example, the CAS number of water is 7732-18-5. The final digit is a check number. CAS registry numbers are used for searching chemical databases. The size of the registry is immense, with over 32 million substances and 59 million sequences. It identifies every chemical that has been described in the literature since 1957 and around 50 000 new numbers are added each week.

https://www.cas.org/support/documentation/chemical-substances/faqs Further information from the CAS site