That "mode" may not be what you are thinking of. The mode here refers to the most common value sampled along the line segment at each node connecting a pair of surfaces or along a normal vector. So if a segment contains values:
1 2 3 6 6 3 7 6
The mode is 6. The maximum, mean, median, minimum will all give different values of course. It might be useful to know how you want to project data from the surface to the volume.