attachment.php - what is stc=1 param / define('NOSHUTDOWNFUNC', 1) ?

[Scandal]

Hello all!
I have detect this code on attachment.php:

PHP Code:

if ($_REQUEST['stc'] == 1) // we were called as <img src=> from showthread.php
{
    define('NOSHUTDOWNFUNC', 1);
} 


... and ...

PHP Code:

    if (defined('NOSHUTDOWNFUNC'))
    {
        if ($_GET['stc'] == 1)
        {
            $db->close();
        }
        else
        {
            exec_shut_down();
        }
    } 


What is 'stc' = 1 param for and what the NOSHUTDOWNFUNC constant does?
Can someone give a small explanation?