What Exists and What is Empty in MDX

After reading my chapter "Managing Context in MDX" in MVP Deep Dives vol2 I noticed that I should have probably discussed one extra topic - the difference between cells which cannot exist, and such which can, but are empty.

The basic idea is that in SSAS cubes we have the notion of empty space. However, there is an important difference between empty intersections which are possible but result in nulls when queried and "impossible" intersections between hierarchies in the same dimension.