Major release: A major, functional release with many new and improved features developed on an 18 month release cycle.
Continuous release (C): A functional release with some new and improved features along with new security updates, bug fixes for existing features, and previously released out-of-cycle patch updates.
Quarterly update (Q): A scheduled update that includes functional improvements, new security updates, and previously released out-of-cycle patch updates. For Reader, these updates are sometimes available as full installers.
Out-of-cycle patch (OOC): An update targeted at security fixes. These noncumulative patch files may contain few functional updates with the intention to limit impact.
Optional Update: An update targeted at a narrow set of functional fixes with limited impact. Adobe recommends that you apply these patches if you are seeing an impact by one of more of the functional fixes being delivered.
Note: Continuous releases, quarterly updates, out-of-cycle patches and optional updates can provide security enhancements. Adobe recommends that all updates and patches be applied when available.