Answer:
In RFEM 6, you can activate and thus display the vectorial display of principal stresses in the Results navigator under Stresses.
Trajectories of Surface Results
For surfaces, you can display the trajectories of principal stresses by selecting the respective angle α for the top, middle, and bottom sides of the surface.
Trajectories of Solid Results
For solids, the trajectories of the principal stresses can be found in the result σ123.
Display
You can control the scale for the display of trajectories by the factor for trajectories in the control panel.