The world Carl must return to, and the threshold Paulie never managed to cross.
The chapter opens on the dungeon as a boundary. Carl asks whether Paulie ever made it inside, and Orren answers that he appears to have been outside but never made it within — most of them never make it in.
Carl himself is only visiting The Memory Simulation; he speaks of jumping back into it and getting back out there, and he wants one last stop with Lightning Lou first. He promises Donut he'll be back soon, then presses Disconnect Containment before he goes, changing what he will be returning to.
The place Carl must return to, and the line Paulie's kind rarely manage to cross.