The section
The vertical slice through a room — the view from the side that shows its full height. A floor plan flattens height to nothing, so the wasted volume above head level is invisible in plan and only appears in section. Designing the section is what unlocks vertical space.
The height the plan hides
Read a room in section before furnishing and you see the idle metre of air above 2 m — paid-for volume doing nothing. In a tall room that question is transformative; in a standard one it still yields a high storage band.