This response means that server could not understand the request due to invalid syntax. The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing). The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications. Wikipedia The request cannot be fulfilled due to bad syntax. General error when fulfilling the request would cause an invalid state. Domain validation errors, missing data, etc. are some examples.