What are the units of amplitude of my ultrasonic recording equipment I'm not familiar with the topic of sound. I have ultrasonic audio recordings sampled at 250kHz (16bit) in WAV format. The equipment used to record them is linked here:
http://www.avisoft.com/usg/usg116Hn.htm

I've use the matlab programming language to plot the audio signal. While the X axis is just going to be the sample number (divide this by the sampling rate to get time), I don't quite know what the units for the amplitude are. 

Typical values (which can be positive or negative) have very high precision and are of the order of 10^-4 or 10^-5. An example amplitude value is:

-3.051757812500000e-05 (at least according to Matlab)

Anyone have any idea of what units I am dealing with? I did not use the equipment myself, my supervisor did. When I asked him what units the amplitude were, he advised I look at the link, but I am not too certain I understand.
Given that the gain of the system and/or ADC seems to be unknown, the scale of the sample values can only be determined from samples of a calibrated source of known value at a known distance from the transducer.  With a variable enough gain knob (or AGC) the scale of the samples could be anything.