Unlike later beta versions, an RC (Release Candidate) is the build Apple intends to ship to everyone. RC1 typically carries the same version number as the final public release. For Big Sur, this was build 20B28 (for the initial release). Its arrival meant Apple had shifted from adding features to pure stability and optimization.
In the fall of 2020, Apple reached a pivotal milestone in the development of macOS 11 Big Sur: Release Candidate 1 (RC1) . For developers and public beta testers, this build signaled that the operating system was effectively finished—barring any last-second catastrophic bugs.