### Fundamental principle
A hierarchical, branching (often fractal) tree network that distributes or collects something—mass, energy, charge, information, or authority—efficiently. Such networks are self-similar across scales, typically form directed acyclic structures, and arise from constraints that favor minimizing transport cost/energy dissipation while maximizing coverage.

- Example (different domain): **Urban water supply network** distributing water from reservoirs through progressively branching pipes to individual homes.