The first step in the digital filter design process is to specify the characteristics that you desire. The more fundamental specification would be the difference equation that is to be satisfied. Such specifications may arise directly from requirements in a signal processing problem.
However, much more common are the specifications that arise when you want to process a continuous time signal digitally, and you expect the digital filter to approximate the performance of an analog filter.
Using the transferFunction block you can design either IIR filters using analog prototypes or FIR filters.