In your custom module, you need
-WF Context entity
-WF Instance Entity (generalization from System Module)-You need for each workflow
-WF User Task Entity(s)(generalization from System Module)( For Example if you have 2 Tasks in WF editor/a WF you need to apart 2 Task entities)
Best
You can use this link
https://medium.com/mendix/mendix-workflow-an-overview-a97aadf7cbc1
A workflow contains the steps of all the processes. Each step in the workflow can have a WorkflowUserTask. Entity is an entity that stores data on the user task execution. So that is something you define yourself.
See also documentation here: https://docs.mendix.com/refguide/user-task
Regards,
Ronald