I just found this out. The author of this particular book is summarizing each "part." When he says something like "In Part 2" .... and the Part 2 is tagged to the actual Part 2 further in the Book you can click and drag the "Part 2" into a new window. This saves time going back and forth and makes it convenient so you can summarize Part 2 in a brief look.
Found this out totally by accident.
mm.