The portion of a UK domain registration that specifies which ISP has control
over the domain (eg, the authority to make changes to which DNS servers are
referenced for that domain).
It consists of an alphanumeric string. Each ISP has a unique tag which
they register with Nominet. In order to obtain a tag, the ISP must have
good credit (as they are invoiced for domain registrations, rather than paying immediately) and have signed
an agreement with Nominet. ISPs who are members of Nominet and have their
own tag qualify for reduced domain registration fees.
Domain registrants can change the tag attached to their domain to transfer it
between ISPs. "Detagging" of a domain, ie removal of the tag, means that
it is not functional until a new tag is attached to the domain, and is
typically done for domains for which money is owing or for which the tagged
ISP is no longer involved with. Registrants can also move the domain to the
NOMINET tag, which means they have control over their own domain details.