hibernate查询中的clazz_属性

Question

在Hibernate表中，当使用超类跟随查询获取时，每个具体类策略生成：

SELECT vehicle0_.VEHICLE_ID    AS VEHICLE_ID1_2_,
  vehicle0_.MANUFACTURER       AS MANUFACTURER2_2_,
  vehicle0_.PASSENGER_CAPACITY AS PASSENGER_CAPACITY1_0_,
  vehicle0_.LOAD_CAPACITY      AS LOAD_CAPACITY1_1_,
  vehicle0_.clazz_             AS clazz_
FROM
  (SELECT VEHICLE_ID,
    MANUFACTURER,
    PASSENGER_CAPACITY,
    NULL AS LOAD_CAPACITY,
    1    AS clazz_
  FROM PassengerVehicle
  UNION ALL
  SELECT VEHICLE_ID,
    MANUFACTURER,
    NULL AS PASSENGER_CAPACITY,
    LOAD_CAPACITY,
    2 AS clazz_
  FROM TransportationVehicle
  ) vehicle0_

在查询中选择了

vehicle0_.clazz_，该表中没有，也没有在课堂上，这个clazz_属性是什么？