The 400 series indicates that the request that the client sending is invalid and not in a format that the server can process:

• 400 Bad Request: This status code indicates that the server is unable to process the request as either the syntax is incorrect, or the value doesn't match the condition that server has defined.
• 401 Unauthorized: This status code indicates that authentication is required to access the requested resource.
• 403 Forbidden: This status code is indicates to the client that it either lacks the required authorization to access the resource, or the server refuses to process the request though the request is valid. The reason will be listed in the response body.
• 404 Not Found: This status code indicates that the requested resource is not found on the server at the location specified in the request.
• 405 Method Not Allowed: This status code indicates that the HTTP method specified in the request is not allowed on the resource identified by the URI.
• 408 Request Timeout: This status code indicates that the client failed to respond within the elapsed time set on the server side.
• 409 Conflict: This status code indicates that the request cannot be processed or completed as it conflicts with the constraints defined on the resources in the server. For example, a resource creation request that already exists can return 409 Conflict, indicating that the resource is already present on the server side.