The program calculates an Exposure factor Ce using NBC 4.1.7.3 (5) based on the terrain category input in the Site Information dialog.
Sentence (c) and the method it references in Commentary I -12 allowing you to interpolate between rough and open terrain is not implemented. 4.1.7.3.(5) refers to the "upwind" direction, but it not possible to calculate a different exposure factor for wind from one direction and another for the opposite direction or transverse direction, the Ce applies to all walls on the structure.