eg. org.apache.maven, org.apache.commons
A good way to determine the granularity of the groupId is to use the project structure. That is, if the current project is a multiple module project, it should append a new identifier to the parent's groupId.
eg. org.apache.maven, org.apache.maven.plugins, org.apache.maven.reporting
eg. maven, commons-math
eg. 2.0, 2.0.1, 1.3.1