What is “Registrar IANA ID”?

Registrar IANA ID is a unique number allocated by ICANN to its accredited registrars. IANA, with its key role of allocating Top-Level Domains to domain registries around the world, also serves as the manager of the Registrar IDs assignment. A full list of Registrar IDs can be found at https://www.iana.org/assignments/registrar-ids/registrar-ids.xhtml.