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alpha = after all new features have been integrated. This version is run for a decade or more to look for instabilities not detected by short running development/integration testing. The first alpha tag is made after all the features from the feature freeze have been integrated to master. Additional alpha tags are made as needed.
beta = after alpha. Versions used for coupled tuning. The first beta tag is made after the coupled simulation group says alpha testing is finished.
rc = release candidate. Near-final version. Checked for documentation, portability. The first rc tag is made after the coupled simulation group determines beta testing is finished AND after a code freeze. The Typically, the "rc" versions and released versions are nearly identical.
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maint-x.y: A branch started when a vX.Y release (internal or external) is made and used to provide updates and bug fixes for that release. A.k.a "release branch". After the maintenance branch is made, master can proceed to the next version.
Examples:
maint-1.0 - contains updates and bug fixes to v1.0
maint-1.1 - contains updates and bug fixes to v1.1
maint-2.0 - contains updates and bug fixes to v2.0
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For current and upcoming tags see Existing and Planned Tags and Branches
The figure below illustrates the relationship between tag names and code development history.
Every circle represents a commit of some code.