We measure elevations from the base to the top of a component. It’s done the same way that we measure the height of mountains from sea level. All components are 1 pixel thick.
The amount of elevation decides the shadow size. In our system, we have two shadow tokens to show the elevation -
The higher elevation casts a bigger shadow with more spread; we call this the 'High shadow'.
The lower elevation casts a narrower shadow, we call this the 'Low shadow'.