The server is denying access to the resource as a consequence of a legal demand.
The server in question might not be an origin server. This type of legal demand typically most directly affects the operations of ISPs and search engines.
Responses using this status code SHOULD include an explanation, in the response body, of the details of the legal demand: the party making it, the applicable legislation or regulation, and what classes of person and resource it applies to.
The use of the 451 status code implies neither the existence nor non- existence of the resource named in the request. That is to say, it is possible that if the legal demands were removed, a request for the resource still might not succeed.
Note that in many cases clients can still access the denied resource by using technical countermeasures such as a VPN or the Tor network.
A 451 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls; see RFC7234.