A model is a collection of related design elements; these elements can include requirements, use cases, objects, diagrams, and other artifacts of design. In the VisualAge team library, a model is stored within the context of a Smalltalk application.
All of the modeling work connected with designing a system is within the context of a model, or a set of related models; a model can have another model as a prerequisite. Model elements in a prerequisite model are visible, meaning they can be accessed (and linked to) from the model specifying the prerequisite. Models can have "uses" and "used by" relationships to other models.