Taverna releases use the following versioning rules.
1st order: X - A signifact change in the Taverna API, and its internal architecture. Taverna 2.0 will be a significant new release.
2nd order: 1.x- A significant change that includes large pieces of new functionality and also some possible minor changes to the API. Plugins may need their code modifying to continue to work correctly.
3rd order: 1.5.x - A change that affects the key core artifacts of Taverna. Changes these core artifacts require that Plugins need to have their dependencies updated to continue to work correctly, though minimal code changes.
4th order: 1.5.1.x - A minor update, that contains bugfixes that do not affact Taverna core artifacts. A Plugin will be compatible across different versions without requiring any changes.
Taverna artifacts for a given release will follow the 3rd order version of Taverna. So all artifacts in Taverna 1.5.1 will have the version 1.5.1.x. When a new Taverna release is made that affects the 3rd order version number, all artifacts will have their versions updated to remain synchronised.