The HyperText Transfer Protocol (HTTP)
510 Not Extended response status code is sent in the context of the HTTP Extension Framework, defined in RFC 2774.
In that specification a client may send a request that contains an extension declaration, that describes the extension to be used. If the server receives such a request, but any described extensions are not supported for the request, then the server responds with the 510 status code.
510 Not Extended
|RFC 2774, section 7: 510 Not Extended||An HTTP Extension Framework|