Elgg is constantly evolving and growing to suit the expectations and needs of its user base. To make sure all the core developers are on the same path, and allow users to submit feedback for what we have in mind, we follow release guidelines and publish roadmaps. Release guidelines cover what to expect when upgrading to new Elgg releases and roadmaps are lists of features and bugfixes considered for future development. They are flexible--nothing in a roadmap is set in stone as features may be added, delayed, or removed altogether. A roadmap serves as a general guide of what to expect and prompts targeted discussion about future Elgg releases
Official Elgg releases fall into three categories: bugfix releases, minor new feature versions, and major new feature versions. Bugfix releases have three digits in the version number (1.7.2) while minor new feature versions only have two (1.8). Major new features versions are identified by the first digit (2.0).
Bugfix release are to make sure the latest version of Elgg is stable, secure, and bug free. They happen every few weeks and are identified by the third digit in the version number (1.7.2). The higher the third digit, the more tested and stable the release is. Since bugfix release focus on fixing bugs and not making major changes, themes and plugins should work from bugfix release to bugfix release.
New features are introduced in Elgg every few months in minor new feature releases. These versions are identified by the second digit (1.8) and aren't as mature as bugfix release, but are considered stable and useable. Though these releases try to be backward compatible, features are added, unused code removed, and overall improvements are made, so plugins and themes might need to be updated to make use of the new and improved features.
Every few years Elgg undergoes serious changes and a major new feature release is made. These version are identified by the first digit (2.0) and are opportunities for Elgg developers to make major improvements to Elgg. These release are used to clean up and change code, so themes and plugins from older versions aren't expected to work without modification on different major releases.
Roadmaps guide Elgg's development and help to communicate to users what is planned for future versions of Elgg. Visit the Roadmap page for more information.