Physical walls define geometrical boundaries that are represented by an infinite plane. You can add those planes by assigning geometry primitives of the type Half space to the physical wall definition. The Wall interaction mode determines if contact interaction between walls and particles should take place in both direction or only for particles that hit the wall in a positive sense. That means for those particles that approach the wall from the direction the plane normal is directed to.
Please note that walls can only be directed in the coordinate axis directions so far. That is normal directions: (1,0,0), (0,1,0), (0,0,1). The reference point of the halfspace can be chosen freely.