Hi Bjorn,
Let me enlighten you! 
If you Event Handling Components, so classes with @EventHandler annotated functions in them, they will be assigned to a processing group.
As this is a requirement, but we didn’t feel that users should be required to use the @ProcessingGroup annotation, we decided to set up a default.
The default processing group of any Event Handling Component (so when you haven’t specified @ProcessingGroup on class level), is the package name of the component.
Thus Event Handling Components which are contained under the same package name, will be part of the same processing group.
When your application matures, it is however recommended to group your Event Handling Components into sensible processing groups.
This is thus where you’d introduce the @ProcessingGroup annotation on your components to group them together.
I hope this gives you the needed background Bjorn!
Cheers,
Steven