The more I learn about it, the more I feel that I am in the dark. First, there are a number of distinct categories of shade.
Most experts break things down to four types of areas.
Mottled or Dappled Shade exists when sunlight filters through trees' canopies, delivering a total of 4-5 hours of direct sunlight to the garden. Open or Northern Shade is when a building or fence casts shadows but there is no overhead blockage of light. Medium Shade is essentially a combination of the first two descriptions. You get the tree canopies to go with your blockage from man-made objects. Dense Shade exists when tree canopies totally obliterate all sunlight.