Maybe somebody else can answer that, I didn't dwell on that section as I hope not to need it! But she is talking about bubs over 6 months old I think it requires having gotten to know your babies sounds and cries over time.

The main thing that I noted is that she's talking about leaving them cry for only a few minutes in any event, and coming in immediately if they are in way distressed. If you don't know your babies cries well enough then her philosophy would be, I think, assume they are distressed and do not leave them to CIO.