Behind feature flag — contact [email protected] to enable. Custom field values keyed by custom field definition ID (UUID) of a definition with entity_type PurchaseOrderRow or OutsourcedPurchaseOrderRow. Each value matches the definition’s field_type — string for shortText/url, number for number, boolean for boolean, YYYY-MM-DD string for date, or the integer choice id for singleSelect. Unknown definition IDs are rejected with a 422.