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Both Tasks are handled by a WorkItemHandler which implement the same interface.
When modelling your process it does matter. A User Task gives you much more possibilities to set parameters tailored for user interactions:
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org.jbpm.services.task.wih.ExternalTaskEventListener#processTaskState
TaskService
The TaskService is registered by the RuntimeManager to the RuntimeEngine. Magnolia provides it's own RuntimeManager implementation for easy registration of a custom TaskService Implementation. It's purpose is to handle the lifecycle of User Tasks, which is pretty obvious when looking at its interface:
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