Sections are classified as skill, lesson, or theorem. Every section should begin with a summary of what you're going to learn and what you must already know. What follows is a description of each section type.
A "skill" is something that can be taught and practiced. Therefore, it must have a "lesson" and "practice" section.
A "lesson" can exist without being part of a "skill" For example, we could have a lesson on the history of squaring the circle. It's just some interesting stuff with nothing to practice.
A "theorem" consists of the subsections Intro, Proof, and Using the theorem. The Intro describes what the theorem tells us, the Proof tells us why the theorem is true, and Using the theorem gives us practice in applying the theorem to solve math problems.