Does CI take id/value from segment 3 only if i pass values in URI ?

[eluser]Unknown[/eluser]
Please can someone tell me, if code igniter takes value from URI from segment 3 only ? Coz whenever i am trying to pass a value through URI and in segment 4, the pagination does not seem to work. Also i dont understand the following line. Can somebody explain ?

$CI =& get_instance();
if ($CI->uri->segment($this->uri_segment) != 0)
{
$this->cur_page = $CI->uri->segment($this->uri_segment);

// Prep the current page - no funny business!
$this->cur_page = preg_replace("/[a-z\-]/", "", $this->cur_page);
}

Thanks