Since a surface is a 2D object whose thickness is considered a physical property, the results are displayed either on the positive or the negative side.
The position of the positive or negative surface side can be controlled by the local axis system of the surface: the positive surface side is the side in which the positive z-axis runs. The negative surface side lies in the direction of the negative z‑axis. You can activate the axis system of the surface using the 'Display' navigator (see Figure 01).
You can use the '+/- z-Surface Side' option to check the position of the surface sides (see Figure 02). It is important for the definition of foundations with failure or also for a reinforcement layout. The top side of a surface is shown in red, the bottom side in blue.