The Portal Zone is Universe 0 seen in the Courtney's Chaotic Multiverse comic series, it's where all the portals to the rest of the universes are, it isn't as big as the other universes and thus isn't seen as one of the main ones. It has heavy security done by The Portal Patrol to make sure nobody escapes the universe they are supposed to be in. Courtney, Serena, and Jack are the only ones that have access to the zone and are allowed to visit other universes at any time.