From XOOPS Web Application System
Starting with the next major release version, XOOPS will use the Linux Kernel Versioning scheme. These version numbers follow a longstanding tradition. Each version has three numbers, i.e., X.Y.Z.
The "X" is only incremented when a really significant change happens, one that makes software written for one version no longer operate correctly on the other. This happens very rarely -- the jump from XOOPS 1 to XOOPS 2 is a fine example.
The "Y" tells you which development "series" you are in. A stable release will always have an even number in this position, while a development release will always have an odd number.
The "Z" specifies which exact version of the kernel you have, and it is incremented on every release.
The current stable series is 2.0.x, and the current development series is 2.1.x. However, many people continue to run 1.3.x and even 2.0.x releases, and they also continue to receive bugfixes. The development series is the code that the XOOPS Core developers are actively working on, which is always available for public viewing, testing, and even use, although production use is not recommended! This is part of the "open source development" method.
Eventually, the 2.1.x development series will be declared stable and become the 2.2.0 release and a new stable series will then be established, and a 2.3.x development series begun. Or, if any really major changes happen, it might become 3.0.0 instead, and a 3.1.x series begun.