About MarkMonitor
MarkMonitor is a corporate registrar in the United States that manages domain portfolios for large brands. Seeing it in a record usually means the domain belongs to a company that uses a brand-protection service, and the domain normally carries several client and server locks.
What the record looks like
A gTLD WHOIS answer carries the two "Registrar" lines below; RDAP carries the same facts as an entity with the role "registrar" and a public ID of type "IANA Registrar ID". The domain here is a reserved example; the registrar lines read "MarkMonitor Inc." and 292.
Domain Name: EXAMPLE.COM
Registrar: MarkMonitor Inc.
Registrar IANA ID: 292
Domain Status: clientTransferProhibited
RDAP: entity with role "registrar"
vcard fn: MarkMonitor Inc.
publicIds: [{ "type": "IANA Registrar ID", "identifier": "292" }]Country-code registries do not use IANA IDs and often print a shorter or local form of the registrar name, or a registry-specific handle. Some publish no registrar at all.