Instance specifications in UML

In UML models, instance specifications are elements that represent an instance in the modeled system. When you instantiate a classifier in a model, the instance specification that you create represents an entity in the modeled system at a point in time, similar to a snapshot of the entity. You can model changes to the entity over time by creating several instance specifications, one for each snapshot.

Instance specifications can include the following information about the entity:

The classification of the entity by one or more classifiers of which the entity is an instance
The kind of instance, based on its classifiers; for example, an instance specification whose classifier is a class describes an object of that class, while an instance specification whose classifier is an association describes a link of that association
Specific values of the structural features of the entity, which are represented by slots

Like classifiers, instance specifications have attributes that are referred to as slots. An instance specification can have one slot for each structural feature of its classifier, including inherited features. You can specify values for each slot in an instance specification as long as a valid type is defined for the slot.

The following figure shows a class named Member. The class has two attributes, ID and Name. The ID attribute is of type Integer and the Name attribute is of type UnlimitedNatural.

The following figure shows the corresponding instance specification that has two slots and values. The value of the ID slot is 1, and the value of the Name slot is John Q Public.

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