Maxwell Surface Stress Tensor

Hi all,

I'm trying to calculate the magnetic attraction force between two objects in COMSOL 4.3a, by using Maxwell Stress Tensor.

I found four possible Maxwell functions to calculate the force:

Maxwell upward surface stress tensor,
Maxwell downward surface stress tensor,
Maxwell upward magnetic surface stress tensor and
Maxwell downward magnetic surface stress tensor.

Please can anyone tell me what is the difference between them?
Thank you.