Occlusion "is the effect of one object in a 3-D space blocking another object from view."
With this definiton in mind I cannot understand sentences like:
The interior of a tube is typically more occluded (and hence darker) than the exposed outer surfaces, and the deeper you go inside the tube, the more occluded (and darker) the lighting becomes.
Occlusion is for me a question of yes or no. Yes an object/point is in front of another, or not.
So how can some points be more or less occluded then others? I think I need another definiton for occlusion.