In HTTP/1.0 and since,
the first line of the HTTP response is called the status line and includes a numeric
status code (such as "404") and a textual reason phrase (such as "Not
Found"). The way the user agent handles the response primarily depends on the
code and secondarily on the response headers. Custom status codes can be used since,
if the user agent encounters a code it does not recognize, it can use the first
digit of the code to determine the general class of the response.