HTTP Status Code 406 signifies Not Acceptable. This status is returned when the server cannot produce a response matching the criteria specified by the Accept headers sent in the request. Essentially, the client has requested a resource in a format that the server cannot provide. For example, if a client requests a resource that can only be delivered in JSON format but specifies that it only accepts XML, the server would respond with a 406 Not Acceptable. This response informs the client that the requested representation is unavailable in the desired format.