Where is the security section be called?

Sorry, my English is poor. I mean the security module in sapphire. I checked the security.php file, I just want to know where the core use security class, do you understand what I say?
see the pic attahced.

Yes，I want to know that. And I found that the security class is used at:
if (Director::isLive()) {
    error_reporting(E_ALL ^ E_NOTICE);
}

but I' m not sure if I understand the exact meaning of the security purpose, because that the annotation of the security.php file tells that this class is used to "Implements a basic security model". What is a security model? Where dose it begin? Does it begin at if(Director::isLive) since this class has used some method of class security?