Every plant lives in a niche in whatever eco-system it finds itself in. Some plants, like tall trees that don't take well to pruning, will only ever be found in one sphere of an eco-system. Such trees will always be a part of the Canopy level of an eco-system. Other plants can sometimes take to the Canopy or stay lower down in the shrub layer.
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Agroforestry
Forest Gardening often refers to these eco-system niches as layers.
Relativity of terms
The terms are relative to the ecosystem. The canopy could be made up of tall trees, or small herbaceous plants.