difference between stress pattern and stress lines

Hey yall,

    i have set a surface to which i apply a load and support, analyse the stresses and proceed to thickening it. The issue is that i now would like to tessellate it using lines that are inherent to the stress analysis but i do not seem to understand the difference between the two components "stress lines" which shows the principal stresses at give points on the surface and "stress pattern" which seems to be applying a sort of grid to the surface, maybe if someone can explain the logic that generates the pattern? 

Thanks a lot

Gabriella