Definition contributed by Chiradip Mandal.
An ultra-large-scale system has a bounded capability surface — a finite, enumerable set of admissible operations, fixed by specification — and unbounded capacity — no architectural ceiling on nodes, load, geography, or lifetime, and growth that requires no change to the capability set.
Additionally, it qualifies as engineered rather than merely large only if its safety (no admissible event sequence reaches a prohibited state) and liveness (every accepted request terminates) properties are stated as machine-checkable invariants and measured continuously in operation, with bounds that hold uniformly as capacity grows.
Where the DoD/SEI treatment casts ultra-large scale as an emergent condition to be managed, this casts it as a design contract.