The Useful Difference
QUAD separates jobs that are often blurred together. Accounting, work proof, passage, and fast movement can coordinate without pretending they are the same kind of truth.
Each surface should publish what it can prove, what it cannot prove, and which next gate would change the label.
A receipt, route, listing, balance, or feed means only what the owning surface can support.
Closed, stale, refused, and not-yet-public states are product signals, not embarrassment to hide.
Core, Infra, Bridge, and Liquid can help each other without inheriting each other's authority.