When you add a reusable task to a workflow, you add an instance of the task. The definition of the task exists outside the workflow, while an instance of the task exists in the workflow.
You can edit the task instance in the Workflow Designer. Changes you make in the task instance exist only in the workflow. The task definition remains unchanged in the Task Developer.
When you make changes to a reusable task definition in the Task Developer, the changes reflect in the instance of the task in the workflow if you have not edited the instance.