  In this notes, we illustrate why the infinite volume scattering amplitude is
in fact dispensable when it comes to formulating few-body quantization
condition in finite volume. Only subprocess interactions or interactions
associated subprocess amplitudes are essential and fundamental ingredients of
quantization conditions. After these ingredients are determined, infinite
volume scattering amplitude can be computed separately. The underlying reasons
are rooted in facts that (1) the final physical process is generated by all
subprocess or interactions among particles; (2) the ultimate goal of
quantization condition in finite volume is to find stationary solutions of
few-body system. That is to say, in the end, it all comes down to the solving
of eigenvalue problem, $\hat{H} | n \rangle = E_n | n \rangle $.
