Each time you select "Add Shift Register", you are creating a separate one - each with it's own terminal on the left and right side of the while/for loop. When you have an existing shift register and select "Add Element" you create a single terminal on the left side and is part of the existing shift register. This new terminal is the previous value of the shift register. So the topmost terminal is the current value and each element as you go down is a previous one. A simple demonstration would be to create a while loop and wire the loop index to the shift register input on the right side and then add elements a couple of times. Create indicators for each and run the VI.
