It might be helpful to describe very similar functionality scenarios multiple times on different levels.
- In a Domain model, describe scenarios without taking into account the designed solution. i.e., how the scenario currently works
- In Boundary model (Design model), describe scenario taking into account the system (i.e., what will the system do)
- In Internals model (Design model), the scenario from boundary model can be refined to include what internal of the system will do (i.e., how will the system do that)