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)