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Alpha and beta releases
When we want new features to get wide testing before we enter the formal stabilization period, we'll sometimes release alpha and beta versions. There is no requirement to do any beta releases even if there were "alpha1", "alpha2", etc releases; we could just jump straight to "rc1". However, there are circumstances where a beta can be useful: for example, if we're unsure of a decision and want to get wider user feedback before solidifying it into a formal release candidate.
Alphas and betas are only for people who want to help test, and who understand that there may be incompatible changes before the final release.
Breaking compatibility
Our compatibility rules only start to apply once a final x.y.0 version has been blessed and released. Any API's, wire protocols, and on-disk formats that appeared on trunk or in an alpha/beta/rc pre-release are not subject to any compatibility promises; we may change them arbitrarily until the final release, if we find a good reason to do so.
In particular we might not provide an upgrade path for persistent data with the final release. Consequently, pre-releases should never be trusted with any data meant for long-term safe-keeping.
At the same time, we wish to remind the reader that the best time to point out API design bugs is before they are released and set in stone during the initial design and pre-release stages.
(Thanks to the Subversion team for the content we 'borrowed' from the Subversion release guideline)