Curtains are also used for privacy and security. They act as a functional covering for an enclosement or a room. Come to think of it: Curtains are used inside the bathroom, curtains are also used in private bedrooms, curtains are also used in the kitchen. For most people privacy is a primordial concern. Curtains prevent other people from peeking right into your personal space.
Your question above seems to be somewhat vague, but would like to see. if we can flush that out a bit. A solution document may be a. Solution Engineering, Solution Architecture, Solution. Integration, Technical Solution, Solution Design, or Solution. Build document.
A topic is to a document what a subatomic particle (such as an electron) is to matter. It is the basic component in a document. Each topic can and must stand alone. But it strikes me as odd to say in one breath that a topic is a basic component of a document, and in the next that it must stand alone.