Explanation
Explanation is a discursive treatment of a subject, that permits reflection. Explanation is understanding-oriented.
Explanation deepens and broadens the reader’s understanding of a subject. It brings clarity, light and context.
The concept of reflection is important. Reflection occurs after something else, and depends on something else, yet at the same time brings something new - shines a new light - on the subject matter.
The perspective of explanation is higher and wider than that of the other three types. It does not take the user’s eye-level view, as in a how-to guide, or a close-up view of the machinery, like reference material. Its scope in each case is a topic - "an area of knowledge", that somehow has to be bounded in a reasonable, meaningful way.