The HTTP 510 Not Extended server error response status code is sent when the client request declares an HTTP Extension (RFC 2774) that should be used to process the request, but the extension is not supported.
In the following example, a client sends a request with a mandatory extension specified in the C-MAN header. The Connection header specifies that these extensions are to be handled on a hop-by-hop basis. A proxy forwards the extended request, but the Connection header is stripped out in transit. Because the origin server doesn't receive any information about the M-GET method, it sends a 510 in response:
M-GET /document HTTP/1.1
Host: example.com
C-Man: "http://www.example.org/"
Connection: C-Man
HTTP/1.1 510 Not Extended
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