Public Suffix List
A maintained list of the suffixes under which the public can register domains, used to find the real registrable domain.
Not every dot marks an organisational boundary. In example.co.uk the registrable domain is example.co.uk, not co.uk, and no simple rule derives that: it has to be looked up. The Public Suffix List records these suffixes so software can find the boundary correctly.
Getting this wrong has real consequences. Reputation applied at the wrong level either leaks across unrelated sites sharing a suffix, or fails to group the subdomains of one organisation together.
Our domain engine resolves the registrable domain through the list before applying any reputation, so a signal about one site never spreads to everyone else under the same public suffix.