From 'Agile Web Development with Rails 4' pag.272.
If an incoming request has a controller named (say) admin/book, Rails will look for the controller called book_controller in the directory app/controllers/admin. That is, the final part of the controller name will always resolve to a file called name_controller.rb, and any leading path information will be used to navigate through subdirectories, starting in the app/controllers directory.
Imagine that our program has two such groups of controllers (say, admin/xxx and content/xxx) and that both groups define a book controller. There’d be a file called book_controller.rb in both the admin and content subdirectories of app/controllers. Both of these controller files would define a class named BookController. If Rails took no further steps, these two classes would clash. To deal with this, Rails assumes that controllers in subdirectories of the directory app/controllers are in Ruby modules named after the subdirectory.
My question is: how could the two book_controller.rb files clash?
I have two different URLs: ..../admin/book and ..../content/book, how can they clash? In the previous paragraph it explicit says
See Question&Answers more detail:os[..] any leading path information will be used to navigate through subdirectories, starting in the app/controllers directory