There is a new edition of the book and this is an old link.
The Rust project uses a concept called ‘release channels’ to manage releases. New nightly releases are created once a day. Every six weeks, the latest nightly release is promoted to ‘Beta’. At that point, it will only receive patches to fix serious errors. Six weeks later, the beta is promoted to ‘Stable’.
This chapter does not exist yet in the second edition. You can check out other resources that describe release channels.