HTTP Status Code : Indicating the general category of the error (4xx for client errors, 5xx for server errors). This example of an error from the Signature APIopens in a new tab includes a type , which is basically the same as an application-specific error code, but instead of an arbitrary string like invalid-api-key the standard suggests a URI which is unique to your API (or ecosystem): https://signatureapi.com/docs/v1/errors/invalid-api-key . This allows the error to be used in a web interface, where certain errors are caught and handled internally, but other errors are passed on to the user to help errors be considered as functionality instead of just "Something went wrong, erm, maybe try again or phone us".