I've been wondering lately if the already established realms: The realm of Light, realm of Darkness, Sleeping Realm, the Datascape, and the realm between, are all technically separate "worldlines". The term "worldline" was something we've only heard mentioned in Luxu's reports, so it could be that it's a term that he understood to be the difference between all these various realms, but was unknown to anyone in the present. Essentially the concept of "worldlines" is lost knowledge and therefore no one knew to use that term anymore.