When you design a course, you think about goals, exams, assumptions about first-day student knowledge, and so on. Hopefully, you write your thoughts down.
It would be convenient to keep that information with the course it applies to. So, when someone else works on the course, they know where to find your documentation. In fact, it might not be someone else, but you two years from now. You'll have forgotten the reasons you decided to do this and not that. You'll need a reminder.
Skilling has a simple content type called a "design page." It's for documenting course design, keeping the information separate from lessons and other types of content. Design pages can be organized in their own document tree, separate from the lesson tree(s), but of the same form.
Authors and administrators can create and edit design pages. Instructors, graders, and reviewers (e.g., accreditation personnel) can see the documents, but not change them. Students can see them or not, depending on whether you tell Skilling to allow it.