I've looked but not found if it is possible to define infill differently for different parts of a model. I have a component where the infil can be quite low to speed up printing but in certain areas higher density is needed for strength. My example is a solid cube but with various projecting bosses. The infil to the cube can be very low but the bosses need to be strong. In the component the bosses are in different height locations so a simple definition of density by height would not work.