A Use Case Specification is a textual description of the functionality provided by the system. It captures actor-system interaction. That is, it specifies how a user interacts with a system and how the system responds to the user actions. It is often phrased in the form of a dialog between the actor and the system. The use case specification is represented in the use case diagram by an oval, and is what most people think of when they hear the term use case.

A Use Case Realization describes how a use case, which is logically defined by the use case specification, is physically implemented within a design model in terms of collaborating objects. It organizes the design model artifacts related to the use case. It often comprises multiple design artifacts such as a class diagram, object diagram, sequence diagram, etc. that describe how the physical design will implement the use case specification.

The purpose of use case realization is to separate the concerns of the system stakeholders, which are typically captured by the use case model and system requirements, from the concerns of the system designers. In doing so, the designers can choose to implement the use case specification without affecting the use case specification.