OptionalColumnsExtra or custom columns on MJ: Content Items (validated against entity metadata; framework-owned columns are refused).
OptionalCompleteNothing further needs to happen to this item.
OptionalExtensionsAnything a later stage in the same run should see, namespaced by the driver.
OptionalFieldsWell-known fields the listing determined directly.
OptionalRequestExplicit request for downstream pipeline work on this item.
The item's stable identity at the source (usually its URL); matches a re-discovery to its row.
One piece of content a driver found.