A building section cuts through the whole structure. A wall section cuts through one wall at a larger scale. A section mark on the plan shows where the cut is.
A wall section shows each layer from outside to inside. A masonry sub sees the veneer, the air space, the insulation and the backup wall.
Use sections for heights and layers that plans cannot show. A plan gives you the length of a wall. The section gives you the height and the build.
Follow each section mark from the plan to the sheet it names. Check that the heights agree with the elevations.