Back in 2009, PlantUML used a simple and unique increment number for its versioning. The major reason is that PlantUML is developed with continuous integration in mind. That means that there are new releases quite often, with (hopefully!) backward compatibility. If the language is usually backward compatibility, it is not the case for the PlantUML Java API itself. The single number versioning schema does not allow to clearly indicated Java API changes. Note that here are two different things :
The PlantUML Language (that is everything that is between @startuml and @enduml for instance) : This part is very stable. We really take care about backward compatibility here. Even diagrams that have been written by earlier versions of PlantUML are still readable by latest releases.
The Java API Integration : it has been changing since the beginning. This impacts only Java Plugins and not tools that are using PlantUML through command line.
Official Versioning Scheme
PlantUML follows now some usual and almost standard pattern based on three numbers : MAJOR.YEAR.RELEASE. If you prefer, you can also read this as MAJOR.MINOR.PATCH.
This number will increase when there will be an incompatible change in the Java API. Also, if ever there is a change in the PlantUML language itself which break ascending compatibility, we will also increase this number, but we expect that this will never happen (it has not since 2009!). Change in MAJOR number will be documented: we will clearly explain what has changed between major versions. We expect less than one change per year here.
YEAR / MINOR
This number will be the releasing year of the version. While this is not completely standard, we think that it will help people to know whether they are using a recent version of PlantUML or not.
RELEASE / PATCH
This number will be simply incremented for each release of the year. We expect about 20 to 30 releases per year. It will be reset to zero at each year change.
Note that when incrementing the MAJOR number, YEAR will stick the the current year, and RELEASE will be also incremented.
Knowing the exact version you are using
If you want to know the exact version you are using, you can type in the command line:
java -jar plantuml.jar -version
or use the following small special diagram description :