For the US department of state visa application, there is a question about the passport book number, which is apparently different than the passport number. What is that referring to? The explanation in the side bar was not very clear and other searches gave answers that seemed to say it was the same as the passport number.
It would be helpful to have an answer that shows a few example passports from different countries with the book number location marked. I'm also interested in understanding exactly how these numbers are used in correlation with the passport number.