A controlled front door (inbound) and publication layer (outbound).
A deliberate boundary for data in and out: a controlled front door for ingestion (inbound) and a publication layer for what leaves (outbound).
Systems rot at their edges. Not in the middle, where you're careful — at the doors, where a screenshot lands in the wrong folder, an export sprawls across the tree, and six months later nobody can say what's source and what's residue.
An exchange layer makes the doors explicit. Inbound has a front door: things arrive in one known place and are routed deliberately, so nothing enters by accident. Outbound is a publication layer: what leaves is generated, bounded, one-way.
The payoff is that the inside stays clean. When every crossing happens somewhere named, the vault never fills with things that merely drifted in — and that's the difference between a system that stays trustworthy and one that slowly becomes a junk drawer.