perception
Figure/ground
The brain automatically splits a scene into the thing being looked at (figure) and the backdrop it sits on (ground) – designers push and pull that boundary deliberately.
When a modal opens and the page behind dims, that’s figure/ground being manipulated: the dimming pushes everything else into “backdrop” so the dialog becomes the unambiguous subject. Elevation, shadow and blur do the same job at smaller scale – they tell your eye which layer is the point right now.
The practical test: at any moment, can you say what the figure on this screen is? If two panels both claim to be the subject, the design has a figure/ground fight – which is really a hierarchy problem wearing different clothes.