Stacking Functions is a concept from permaculture. It is done by an element of a system which operates more than one function. Some examples are: a chicken, a tree, ... or even me.
A chicken produces eggs, feathers, and maure; it controls insects and weeds; ventilates and prepares soil before planting; and produses body heat in the mean time - which we can even use with smart design.
A tree photosynthesises; shades understory; holds wind and humidity; breaks up compacted soils; produces organic matter, fruit, building material, and firewood; protects from erosion; and not least, it emits Oxigen. This list can easily be continued, since trees have breaking records in multifunctions.
Or me... You can get more information about what kind of functions I fill in this life, by browsing through this site. I wish you pleasant and useful clicking!