Each section is a "world" - that is, a collection of locations that are related to each other but not to any location in any other "world". The first is Our World, and after that they're in alphabetical order.
Many books contain a version of Our World. For books where Our World is not part of a larger "world" (e.g. Dogsbody, Eight Days of Luke, Fire and Hemlock, Power of Three), the locations for that book should be listed following the nested list rules in the "Our World" section, with the title of the book included in parentheses afterwards. For books or series where Our World appears as only one of several worlds (e.g. Chrestomanci series, Deep Secret, Howl's Moving Castle), details should be listed under "Our World" in the appropriate local section.
As of The Lives of Christopher Chant there are twelve Series, each containing nine worlds - except Series Eleven, which contains only one. (There are hundreds of other worlds, but they are rarely if ever visited by anyone from the Related Worlds, so nothing is known about them.)