The version is the one specified when deploying the application through the manager.
See the documentation:
tag: Specifying a tag name, this allows associating the deployed webapp with a version number. The application version can be later redeployed when needed using only the tag.
Also, you can deploy multiple versions of the same war by adding the version after ## (e.g. myApp##1.0.war, myApp##1.1.war). The manager will show this version in the overview.