Partially update one of your line items in place, for example the final picked quantity and cartons. Fields not sent stay unchanged and an explicit null clears a field. The line item keeps its uuid, and its links (purchase order, sku, container, allocations, routings) are left as they are; the quantities recorded on a routing's allocations are not changed. Sending uuid or a link field is a 422; custom columns are written through /custom_column/upsert_custom_column_value_for_record/line_item_fulfilled/{uuid}. A PATCH that changes nothing writes nothing.