In reality shade is a much more diffuse concept.
First, there is no such thing as "one-size fits all" shade plants. Plants have varying degree of shade tolerance ranging from those which grow best in mostly sun to those which do well even in a dark corner of the yard. Just because a catalog groups several plants as "shade plants" it does not mean they perform equally well under all circumstances. Some plants will grow adequately in a fair amount of shade, but may need staking to avoid leaning. Some plants prefer protection from hot direct sun to avoid dehydrating. Other plants refuse to flower properly without direct sun for a substantial part of the day.
Shade is not a definitive concept.
Shade can be light or dark depending on how much sun gets through to the leaves of the plants. A mesh screen or the canopy of a tall tree may permit varying degrees of sunlight through. The garden may receive direct, unobstructed, sunlight for part of the day, but be shaded by buildings, fences, or trees during other times of the day. A flowerbed may be in full sun in spring before the trees leaf out, but be covered by heavy shade in autumn. A tall oak tree with a loose canopy may allow more sunlight to reach a plant compared to a low growing maple with a dense canopy.
Shade tolerance vary by location
Sunlight intensity is typically weaker in the morning and in the late evening and much stronger during midday and the early afternoon. A shade tolerant plant can thus be planted in morning sun, but afternoon shade, whereas a less shade tolerant plant could receive morning shade and still thrive. Sunlight is also stronger the further south you go. Plants which require full shade in the dessert may require full sun in the northern half of USA to perform adequately.
Water and wind matters, too.
To make it even more difficult, shade is sometimes defined by the amount of water reaching the plant. A low growing maple tree may prevent rain from reaching the ground under the canopy, and the surface growing tree roots will compete for water and nutrients. This is also called dry shade. These conditions may not favor the growing of moisture loving species, such as ferns, for example, whereas a north east corner behind a tall fence receiving the same amount of sunlight may work wonderfully because water is available and there is no competition from tree roots. This is also commonly referred to as wet shade, especially in areas where the soil never dries thoroughly due to lack of sunlight and wind.
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