There are two ways to do it, depending of the Surface mode (XY or UV):
- XY mode: the items are scattered using the XY plane of the Forest object (in local coordinates), and then placed on the surface moving each object along its Z axis. You would create the object as usual, and then rotate it together the spline/surface, to align them with the wall.
- UV mode: uses the mapping coordinates of the surface as alignment guide for the Distribution map. You can use the wall as surface directly, although on this mode some features are not available (as the Areas).