The unit of calculation is 10V. These 10V are provided by the voltage reference board. We are using two 10-turn potentiometers to dial in the input variables: 0.4 and 0.3. Since the adder inverts the sum, we apply another adder to revert the negation.
So in the end we find that the sum of 0.4 and 0.3 is ... somewhat around 0.686.
As you can see, you only get an approximation to the real result. You really need should know the errors of all the involved controls, circuits and meters, calculate the resulting error and specify it with the calculation result in order to make any sense of it.