Operates on Service Orders, shown in the Zuper client app as "Work Order" (in every industry). Work Orders share the exact same underlying schema and endpoints as Purchase Orders / Material Orders; they're distinguished only by the purchase_order_type field (SERVICE_ORDER here) and the /service_orders route prefix. Status transitions are validated against a fixed transition matrix — an unreachable transition is rejected with 400. WORK_COMPLETED is not a valid status for a Purchase Order / Material Order; use FULFILLED instead.