A Registry Number (RN) looks like this:
123456-78-9where the first segment can be from two to seven digits long, followed by two digits, then a single check-digit. It is a sequential accession number from the CAS Registry database, and it carries no chemical or structural meaning in itself. It is simply an identifier for a specific substance that CAS has registered during the process of indexing the literature (or added from another source). The shorter the first segment, the older the registration and the more common (and probably better described) the compound is.
(Medline [Ebsco and ISI versions] also has an RN search field. However, Medline/PubMed uses a more general RN indexing policy that is not nearly as precise as CAS' and it can cause confusing results.)
Most non-bibliographic chemical databases also allow searching by Registry Number.
See the CAS Registry System to learn more about Registry Numbers and their importance.