The sample article tries to have one of everything, plus a few torture tests. It is a good place to test initially, especially with the tcb-style templates. The sample book is less haphazard, but does have most of the structure a typical book would have. So if you are designing for a book it is a good place to test page styles, headers and footers, and division titles. (2019-09-30: it is possible <part> has not been tested thorougly enough yet.)