It is used as reference concept in scientific hierarchy definitions, and many technical approaches, like the tree in computational data structures or nested set model of relational databases.
[2] In particular, when scanning all pairs of subsets at the second condition, it is true for any combination with B.
Using a set of atomic elements, as the set of the playing card suits: The second condition of the formal definition can be checked by combining all pairs: There is a hierarchy that can be expressed by two branches and its nested order: B3 ⊂ B2 ⊂ B; B1 ⊂ B.
[3] The concept of nesting is exemplified in Russian matryoshka dolls.
Illustrating the general concept: A square can always also be referred to as a quadrilateral, polygon or shape.
Nested hierarchies are the organizational schemes behind taxonomies and systematic classifications.