An  Adjacency Item is a member of a Kuratowski's ordered pair, and represents an elementary constituent of a model, used to reference an Element. It is a basic construct to form Kuratowski's ordered pairs.

An  Adjacency Item is either a Reference Adjacency or a Nesting Adjacency .

A   Class of Element is a collection of Elements which it classifies according to one or more criteria.

In knowledge representation, a  Class of Element can be defined either by extension (specifying its members), or by intension (specifying criteria), using what is called "ontology" in languages like OWL.

Not two  Class of Elements have the same membership. However, a distinction must be made between a  Class of Element having members, and those members being known, so within an information system the members recorded may change over time, even though the true membership does not change.

A  Nesting Adjacency is an Adjacency Item that references its Element by nesting.

Nesting is a fundamental concept in complex systems organization where Entitys are contained within other Entitys, creating hierarchical or web-like structures. This embedding mechanism intrinsically links the properties, behaviors, and lifecycles of nested Elements to their containing Elements, forming cohesive units at multiple scales.

Nesting facilitates the emergence of complex aggregates and properties from simpler Entitys, enables efficient design of systems across levels, and supports both autonomy and integration within systems.