Answer the question. Withhold the document.
A verifier rarely needs your identity — they need one fact about it. The engine can issue credentials that answer exactly the question asked and nothing more: age thresholds without birthdates, residency without addresses, entitlement without enrollment history. The verifier receives a compact zero-knowledge proof that the claim is true of a live, biometrically verified person. The proof withholds the underlying personal details; issuer records and service metadata have separate handling requirements.