patch_unset_redactable_field· reason_code· applies to event_envelopeAn `ak.schema.patch.v1` `$op="unset"` addressed a redactable content-carrier slot. The normative path set is registry/redactable-field-registry.json (Message / Morph content pairs; Strand Description and synthesis content pairs) plus any Realm-schema field marked `redactable: true`; the description here is not the criterion. Absence of a content slot on the materialized object is reserved for `never authored` and `cleared by redaction`, so an ordinary update MUST NOT remove it. This is a slot-existence rule, not a capability boundary: `$op="set"` on the same path is ordinary authoring and MUST be accepted even when the new value carries an empty body, and no patch op can reproduce the whole-object, terminal, audit-committed effect of redaction. `metadata`, `encrypted_metadata`, `metadata.title`, `metadata.summary` and paths under `metadata.fields` are NOT covered and MUST accept `$op="unset"`. See zh/models/event-and-patch.md §4.2.4.