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The release number consists of three digits separated by decimal points, followed by an optional status e.g. 2.1.6-beta2. The first digit is the major release number; the second is the minor release number; the third is the patch number. The optional status is an indicator of the release status, e.g. -alpha2, -rc1, etc. Generally, the status indicator is only used for major releases, e.g. 1.0.0-rc1 but is available for any release at the release manager's discretion.
SNAPSHOT Releases
During development of a release, nightly builds are typically done and published under the SNAPSHOT rubric. The SNAPSHOT will contain the latest result of building the release, regardless of the release status. For example, while developing release 2.1.6, the version 2.1.6-SNAPSHOT will be the latest result even though the status changes from -alpha, -alpha2, -beta, -beta2, -rc1, -rc2. The last nightly before release would still be 2.1.6-SNAPSHOT.
Backward Compatibility
Backward compatibility means that user programs compiled against e.g. 2.1.5 will execute when run with e.g. 2.1.6. There is no guarantee that the reverse is true, i.e. that user programs compiled against 2.1.6 will run against 2.1.5.
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