Laws of UX · Perception and grouping
Law of Common Region
Elements tend to be perceived into groups if they are sharing an area with a clearly defined boundary.
The design-system take
Where the principle shows up in a token, component or pattern decision.
Cards, fieldsets and bordered panels are the common-region primitive; a border token with a radius is often all the grouping a layout needs.
Adapted from Law of Common Region on Laws of UX — Jon Yablonski.