The Branch Freeze Policy describes the process of building packages during this phase. Essentially there are non-rawhide repositories with the version number of the release-to-be (currently 21). New builds go to the "updates-testing" repository; after approval in Bodhi, they are promoted directly to the "stable" repository for that release. (There is no "updates" repository until the final general availability release.)
Nightly live builds
Nightly builds of all approved Live spins are now available. The maximum size is now 1GB, which is larger than can fit on a CD. It is expected that most users will have USB or DVD media available. USB media in particular is advantageous compared to CDs, because it is cheap and highly reusable.
Builds include only package versions from the "stable" repository, so they should in general be usable; please file a bug if you encounter any problems. If there is a bug in the generation toolchain, the images may not be built on a given night; in this case, the last built image will remain available.
Using these nightly builds is an ideal way to test the Branched release if you have no spare machine or partition available, or simply do not have the time to maintain a Branched installation. It's a very safe way to test, since it will make no changes to your installed system. You can also later install the Branched release to your hard drive from the Live desktop if the Live image is working well for you.
Various daily spins are available, but "desktop" is the one that is distributed most widely.