There are factors to consider apart from whether a book should be produced as a Type 2 or Type 4 DTB. For example, if a book that is to be added to a library's general reading collection contains 5 levels of headings and subheadings, should the structure be complete and include all 5 levels, if it would require many hours of work to create the structure? Would the first 3 levels (headings with levels 1 through 3) be sufficient to provide 98% of all end users with the navigation they would need to use the book efficiently and effectively?