Functions describe how & what any particular ISP contributes to system stability and sustainability of its environment. Function is what a system DOES. It is often its raison d’être (reason for being at all). As such, function is tied to the ontology of the system, or as it is regarded in SPT, its ontogenesis; its rationale for existing.
In engineering, “function” would be a statement of the system’s purpose or what is its goal. Natural science rejects use of the word goal or purpose as anthropomorphic and damaging to the objective approach central to science. However, quite appropriately, SE, focuses a great deal on the “purpose” as do other human pursuits. We suggest the word function to serve both purposes (pun intended). Function can subsume goal or purpose and yet preserve the demand for objectivity espoused by scientists.
The function of a system defines how it fits into and adapts to its environment while ensuring its own continued stability and sustainability. Examples of identifying functions for hierarchy-forming processes would include: (i) reduce total complexity by bounding in subsystems; (ii) increase stability of multi-level assemblies; (iii) employ benefits of modularity; (iv) enable increase of mix-n-match variation; (v) escape upper interaction and general complexity limits (the Wilson-Troncale limit); (vi) allow for redundancy; (vii) simplify top-down regulation; (viii) enable bottom-up control; and more.