core-bundle/src/Framework/ContaoFramework.php
private function initializeLegacySessionAccess(): void
    {
        if (!$session = $this->getSession()) {
            return;
        }

        if (!$session->isStarted()) {
            $_SESSION = new LazySessionAccess($session);
        } else {
            $_SESSION['BE_DATA'] = $session->getBag('contao_backend');
            $_SESSION['FE_DATA'] = $session->getBag('contao_frontend');
        }
    }
private function initializeLegacySessionAccess(): void
    {
        if (!$session = $this->getSession()) {
            return;
        }

        if (!$session->isStarted()) {
            $_SESSION = new LazySessionAccess($session);
        } else {
            $_SESSION['BE_DATA'] = $session->getBag('contao_backend');
            $_SESSION['FE_DATA'] = $session->getBag('contao_frontend');
        }
    }
Initializes session access for $_SESSION['FE_DATA'] and $_SESSION['BE_DATA'].