Zone File

A zone file is the text file a registry publishes containing every active domain in its TLD along with the nameservers for each. The .com zone file is a few gigabytes and lists well over a hundred million names.

ICANN's Centralized Zone Data Service gives approved applicants daily access to the zone files for most gTLDs. Investors and tool builders download them every day and compare today's file to yesterday's. Names that appeared were registered. Names that vanished dropped. That's how a lot of "new registrations" and "just dropped" reports are generated.

What a zone file doesn't have is ownership or expiration data. It's just names and nameservers. A domain that's on clientHold isn't in it either, since it has no nameservers in the zone, which is one way to spot suspended domains in bulk.