510 Not Extended
Meaning
Further extensions to the request are required for the server to fulfill it. The server is indicating that it needs certain extensions (often specified in the request header) to be present.
When to Use
When a client makes a request that is valid but not sufficient because it lacks certain extension declarations. This was introduced by an HTTP extension framework (RFC 2774) to indicate that the server could not process the request without specific extensions to the protocol being declared.
Example
A client might send a request that uses an extension to HTTP that the server recognizes but requires more information about. The server responds 510 Not Extended, perhaps along with a header (like "Required") indicating what extensions the client needs to support or include for the request to succeed.
When Not to Use
Do not use for normal feature negotiation or unsupported features (501 is for not implemented features). 510 is very rarely used and only makes sense in the context of the extension framework which is not widely adopted.
Source
RFC 2774