...
- constants - we hold here classes like AwesomePluginConstants.java and maybe some model enums that are plugin specific.
- hooks - model and view hooks like: ViewNameViewHooks.java, ModelNameHooks.java
- validators - model validators like: ModelNameValidators.java, SomeOtherModelNameValidators.java
- listeners - view listeners like: ViewNameListeners.java
- states - if there is some logic regarding model states (I consider enum state model field as something that common to write it here), for ex. ModelNameStatesService.java
- workPlansColumnExtension - http://wiki.qcadoo.org/display/QCDMESDOC/WorkPlans+column+extension
...