Notes for myself: Shading can be separated into two big groups: Shadow Family and Light Family, the divider between the two is called the terminator. Within the shadow family, the core shadow exists when there is indirect light like reflected light / ambient light (?). Note that the core shadow eats into the shadow family from where the terminator begins, and not encroach on the light family on the other side. Angle and distance matters when dealing with the intensity of reflection. I.e. the reflected light will get weaker the further away from the ground the plane is, like the sphere where it turns away and gets further away, so the reflected light intensity gets weaker. The space nearest to the ground where the sphere contacts the ground will experience ambient occlusion, therefore will be very dark, but transitions into reflected light and thus will have a great opportunity for drama and interest here. In the light family, I suppose the instructors calls the majority of these values here halftones, with exception to the center light and the highlight. Highlights are usually the brightest, and they reflected the light source directly, so its position is dependent on both your viewing position and the form of the object, which means that the highlight will move with you when you reposition yourself, whereas the center light is fixed, dependent only on the position and form of the object, and the position of the light source. The highlight happens when the object is specular / reflective enough to catch direct rays from the light source to reflect back at you, while really rough and matte surfaces like sandpaper won't be specular enough. Since its a direct reflection of the light source, highlights are usually found closer to the center light than they are to the terminator.

Incredible explanations!