A fundamental building block in Permaculture is the “element.” Every element is either a plant, an animal, or a structure. Each element has inputs, outputs, functions/behaviors, and intrinsic qualities.
The “inputs” are things that element needs in order to function or survive. For example, all animals need food, oxygen, and shelter of some sort. All of these are “inputs” in one form or another. A “dirt road” might need gravel in places where ruts are causing erosion, in order to keep the road maintained and reduce or prevent further erosion.
The “outputs” are things that element provides naturally, that may be used by the system. Chickens provide feathers, meat, eggs, and heat, for example. A small flock of chickens in a greenhouse in the winter can help keep the grow space warm on cloudy days. A barn might provide water as a side effect of its intrinsic property of having a roof with gutters that drive water to a storage tank during a rain.
A chicken “functions” as a natural tiller due to its intrinsic quality of needing to scratch at the ground in search of food. Roads “function” as both access to important areas of the property, as well as providing hard run off to direct water to swales or other catchment devices to keep ponds topped off.
Trees provide shade, act as wind breaks, produce food, and lumber, and give shelter to wild animals on the property.
Sometimes qualities of a specific breed might come into play. Some breeds of chicken do well in cooler climates than others. Some lay more eggs. Some produce higher qualities and quantities of meat.
A barn with a steep almost A frame style roof holds up to lots of heavy snow seasons better than a shack with a flat roof.
All of these characteristics are important when choosing when and where to use an element in design.