5×× Server Error
510 Not Extended
Description
The 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.
See Also
Source: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/510