RC as in Release Candidate just notice the user that the ISO is not the final release of the Install Media of Qonos. Manjaro it self is Rolling, so what we push to stable branch gets added to your local install regardless of which Install Media you have used. And we pushed Qonos a while ago to our stable branch. All ISOs added to our download page are based on stable branch anyway.
So if you get 21.1.6 ISO and update you will end up with 21.2rc1.