Please help me understand $this->site_sentry->login_routine()

[eluser]Tony W[/eluser]
I saw below script in the login file of Bamboo Invoice

if (isset($username) && $username != '') { // if they tried to login
$user_id = $this->site_sentry->login_routine();
}

But I don't understand what $this->site_sentry->login_routine() means. Can someone explain it a little bit?

Thanks!