A stack is the full end-to-end instantiation of all 7 strata optimized for a specific domain or use case. It's not just the technologies, but their wiring: which model, which memory backend, which tool set, which orchestration pattern, which UI paradigm, and which governance policies. Stacks are repeatable blueprints — once assembled and battle-tested, they can be cloned for similar use cases. Examples: research loop stack, customer service stack, code generation stack.