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JIRA and Confluence releases are versioned using a "double point" naming scheme. The first digit in the release name represents a family of major releases. The next digit represents a major release number, and the last digit marks the number of the maintenance release. For example, version "3.2.3" is the third maintenance release, of the "3.2" major release.
Maintenance releases, e.g. 3.1.1, 3.2.2, are bug fix releases. These releases do not change the external (plugin) API of their major release, and leave internal API mostly unchanged. The changes in the API would only be introduced if they are absolutely necessary to fix a bug. Therefore, plugins developed for a particular major release should work for all minor releases that follow.
Major releases, e.g JIRA 3.1, 3.2, will likely introduce changes to the internal API and may affect external (plugin) API. Therefore, if an API that a particular plugin uses has been changed, it is likely that the plugin will need to be tweaked for the new major release. All changes to the external API are documented in the Upgrade Guide of the release for JIRA, and in the Release Notes of each Confluence release.