What I was expounding on was a cosmology of parallel worlds to Earth, each with varying magical activity. I did not mention the technology level of those worlds or other parallel worlds in the series, but they too exist in that multiverse.
Aerth was detailed for the Dangerous Journeys game system, but not that envisioned in 1984 as a direct connection to Oerth was no longer possible. Since then I have designed the Learth, the world setting for the
Lejendary Adventure FRPG. It too is separate as the Aerth is no longer my IP either. At least the baseling, Earth, remains available to all designers seeking to devise a connected series of parallel worlds
So many game projects, so little time...
Cheerio,
Gary