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. Returns the set of fields available to filter on for this module, along with each field's supported operators — use this to build filter_rules for the List / generic filter endpoints.