Extensibility Mechanisms in UML.

Basically Extensibility mechanism allows you to extend and customize UML  by adding new building blocks, creating new properties and specifying new semantics in order to make the language suitable to your specific  problem domain.

There are 3 Type of extensibility mechanisms:

1. Stereotypes.
2. Tagged Values.
3. Constraints.

Stereotypes: 

 Stereotypes allow you to increase vocabulary of UML. You can add, create new model elements, derived from existing ones but that have specific properties that are suitable to your problem domain. Stereotypes are used to introduce new building blocks that speak the language of your domain and look primitive. It allows you to introduce new graphical symbols.

For example: When modeling a network you might need to have symbols for router, switches , hubs etc. A stereotype allows you to make these things appear as primitive.

Tagged Values:

Tagged values are used to extend the properties of UML so that you can add additional information in the specification of a model element. It allows you to specify keyword value pairs of a model where keywords are the attributes. Tagged values are graphically rendered as string enclose in brackets.

For Example: Consider a release team responsible for assembling, testing and deployment of a system. In such case it is necessary to keep a track on version and test results of the main subsystem. Tagged values are used to add such info.

Tagged values can also be used to add administrative info such as author name, date modifies etc.

Constraints:

They are the properties for specifying semantics or conditions that must be held true at all the time. It allows you to extend the semantics of UML building block by adding new protocols. Graphically a constraint is rendered as string enclose in brackets placed near associated element.

For example: In development of a real time system it is necessary to adorn the model with some necessary information such as response time.