What is "Vop" sensitivity unit?
I believe it is volts, measured at Vout of the reference circuit in Figure 2. 

That corresponds to the peak voltage relating to the bounce off of the reference object (PVC pole of specified dimensions at 600mm). 

The decay time will determine the closest object you can detect, since the transducer is rung up at the resonant frequency during the transmit (8 cycles at 58kHz).