Here is a document I found from the '06 Lexus IS250/350 new car features handbook. They use A/F sensors on the primary and heated O2 sensors for secondary to monitor the cats.
Air Fuel Ratio Sensor and Heated Oxygen Sensor1) General
* A planar type air-fuel ratio sensor and a cup type heated oxygen sensor are used.
* The basic construction of the oxygen sensor and the air-fuel ratio sensor is the same. However, they are
divided into the cup type and the planar type, according to the different types of heater construction that
are used.
* The planar type air-fuel ratio sensor uses alumina, which excels in heat conductivity and insulation, to
integrate a sensor element with a heater, thus improving the warm-up performance of the sensor.
* The cup type heated oxygen sensor contains a sensor element that surrounds a heater.
2) CharacteristicsAs illustrated below, the conventional heated oxygen sensor is characterized by a sudden change in its
output voltage at the threshold of the stoichiometric air-fuel ratio (14.7:1). In contrast, the air-fuel ratio
sensor data is approximately proportionate to the existing air-fuel ratio. The air-fuel ratio sensor converts
the oxygen density to the current and sends it to the ECM. As a result, the detection precision of the air-fuel
ratio has been improved. The air-fuel ratio sensor data is read out by a hand-held tester.