Figure 1.16: FANOUT with the Toffoli gate, with the second bit being the input to the FANOUT (and the other two bits standard ancilla states), and the output from the FANOUT appearing on the second and third bits. Source: Quantum Computation and Quantum Information: 10th Anniversary Edition (Fig 1.16, p.30) by Michael A. Nielsen & Isaac L. Chuang
Why does the output from FANOUT appear on the second and third bits? How do we extract and use this output? I learned FANOUT with the following definition and diagram. Thus, is the value of $a$ output from our circuit with Toffoli gate the value of $N$ in the above diagram? Isn't $a$ either 0 or 1? Then does it mean the value for fan-out can only be 0 and 1? I didn't think so as it's there are clearly more than 1 gate from fan-out in the above diagram.