Z-Suite lacks the ability to print infill and shells for a model, as it can only print them separately.
This is a a common feature present in other silicer software like Raise3D IdeaMaker and MakerBot Print.
Example image from MakerBot Print:
Printing both has the benefit of relative density.
With something like 6 shells and 10% infill a model with small and large sections, will have alot of density at the small sections from the 6 shells, while larger sections have the 6 shells and 10% infill for structural support.
You can achieve the same thing with high infill, but this results in a lot more material used and a higher print time.