Hi, I have done a few of these, and maybe Peter will weigh in. In the PV side, your units are basically what your input would be whether 4 - 20mA or 0 - 10V. (There are many unit configurations available: -10 - +10V, 0 - 20mA, etc. Check your device information) As you can see, my units for the PV that I had in this project were 4 - 20mA. The engineering units are the range of the device. In this case in my project it was a temperature transmitter with a range of minus 100 degrees F to plus 200 degrees F. This is what will eventually be displayed in other places such as an HMI display and also in logic for analog values and can be used in compare instructions and the like.
So for your application, your level sensor is either mA or Volts. Then the zero to 16 foot would be the engineering units (range) Hope this helps.