Retention Policy. The explicit rule for how long evidence, telemetry, prompts, artifacts, decision records, approvals, and operational records must be kept, in what form, and under whose authority they may be expired, compacted, redacted, or deleted.
Retention policy is architectural because it decides what can still be reconstructed later. Truth and governed evidence often need durable retention; observation can be sampled and bounded. The danger is letting storage defaults decide which facts survive.