The Design Prototyping phase typically uses CAD tools to efficiently iterate on dimensionally-accurate aspects of your prototypes
To be efficient during this phase, it is important to utilize modular design practices - particularly parametric modeling
Freeform Modeling equates to digital sculpting, and is about creating shapes that are more organic, less constrained by straight lines and perfect curves. In AM, this allows for the design of ergonomic and complex shapes that would be difficult or impossible to manufacture traditionally.
Additionally, other modeling techniques such as Surface Modeling can be utilize with great results for non-geometric forms in CAD