Custom error

[anirban_das]

Hello,
i am looking to create custom error messages just like the Duplicate Post error.
I want to know how can i do this. I found the duplicate post phrase in functions_newpost.php

// with ajax quick reply we need to use the error system
if ($vbulletin->GPC['ajax'])
{
$dataman->error('duplicate_post');
$errors = $dataman->errors;
return;

}
else
{
$vbulletin->url = 'showthread.php?' . $vbulletin->session->vars['sessionurl'] . "t=$prevpostthreadid&goto=newpost";

Can anyone help me locate the error hook location? Say i want to add another error code just like this so that when a member tries to post a given field he's disallowed. Please help me out.