It is designed to amplify signals from transducers such as strain gauges, whose impedance varies based on, for example, mechanical strain. These transducers are usually connected in a bridge circuit.
The 3C66 drives the bridge with a 25 kHz "carrier" signal, which is modulated by the relative impedances of the arms of the bridge. The resulting signal is amplified and envelope-detected by the 3C66, producing a vertical deflection proportional to the measured quantity (strain, for example).
The 3C66 provides built-in variable impedances for balancing the bridge. Because the carrier is 25 kHz, the maximum bandwidth of the measured quantity is 5 kHz. This limitation is due to the settling time of the envelope detector. In addition to the vertical deflection signal, the 3C66 provides a front panel output of the measured signal.