What is /local/externserverfile.php ??

I the url resource, lives this bit of code:

function url_get_encrypted_parameter($url, $config) {
    global $CFG;

    if (file_exists("$CFG->dirroot/local/externserverfile.php")) {
        require_once("$CFG->dirroot/local/externserverfile.php");
        if (function_exists('extern_server_file')) {
            return extern_server_file($url, $config);
        }
    }
    return md5(getremoteaddr().$config->secretphrase);
}

What is /local/externserverfile.php that it is trying to find. I can't find it documented anywhere.