Operates on Purchase Orders. In the Zuper client app this module is labeled "Purchase Order" for most companies, and "Material Order" for companies in the roofing industry — the API and data shape are identical either way. Purchase Orders and Work Orders (Service Orders) share the exact same underlying schema and endpoints; they're distinguished only by the purchase_order_type field (PURCHASE_ORDER here) and the /purchase_orders vs /service_orders route prefix. Records fulfillment/receiving progress against a Purchase Order's line items — recomputes fulfilled_quantity, outstanding_quantity, per-item status, and rolls up the order's total_price if unit_price changed. This endpoint exists only for Purchase Orders / Material Orders — there is no equivalent /service_orders/:uid/line_items route for Work Orders.