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. Rule-engine-based list endpoint — an alternative to the simple filter.* query params on the plain List endpoint, for building complex AND/OR filter conditions. purchase_order_type is enforced as a top-level query field (not injected into filter_rules) so it still binds correctly even when filter_rule_operator is "OR".