The place where two eco-systems or habitats meet (e.g. woodland and meadow) is generally more productive and richer in the variety of species present than either habitat on its own. In ecology this is called 'ecotone'.
This is central to the idea of using edges as a design method. The logic is simple. If the most productive bit of woodland is the edge, then design it to have a bigger edge.
These ideas are used in alley cropping, shelterbelts and pond design.
Marginal could be ideas, views, unusual plants, wild animals or people at the 'edge' of society. Permaculture itself has been seen as marginal for many years....