The Blender output processing flow is powerful, and quite logical once you grasp it. It can be confusing for a noob (trust me: "I added a clip in the VSE and now my scene doesn't render").
I think a simplified view of the flow is as follows; hopefully it is correct. The default behavior conveniently hides some of the complexity. I fairly often use "unusual" workflows, and the picture especially helps me then. Maybe this is already illustrated somewhere; I didn't come across it.
