You can create groups in a book file. Like folders, groups are also logical containers in a book file; they do not exist on the disk. The difference between a group and a folder is that a group does not change the hierarchy of the files within it. Also you cannot set up numbering styles for a group.
For example, the chapter number increments sequentially if the chapter is in a group. For files organized under a folder, the chapter number remains the same but the section number can increment sequentially. You cannot explicitly change the chapter number for files in a folder as the files inherit this property from the folder.