The porosity of a cell is the cell area available for over-the-cell routing divided
by the cell area and multiplied by 100. Thus, porosity is the percentage of cell
area that is available for over-the-cell routing. The porosity of a design is the
same ratio for all cells in the design and its subdesigns.
Когда Вы ставите min_porosity, компилятор пытается оптимизировать схему, чтобы average porosity не нарушал этот констрэйн. Про max_porosity я не нашел ничего в solvnet, наверное это от старых версий осталось