Why 18 and not another number greater 12?
Inside a whitepaper released with the announcment the version 18c is not named at all.
The whitepaper just contains some high level information about the autonomous database:
If this new database works like promised:
Fully automated patching, upgrades, backups, and availability architecture perform all routine database maintenance tasks—without the need for human intervention.then there is no need for documentation at all...