Extensibility mechanism

Refer to stereotypes, tagged values, and costraints.
It provide the ability to customize UML diagrams.

Stereotypes - identify a set of qualities that may appear on a number of diagram elements.
Tagged values - allow you to add new features to an element of the diagram
Constraints - define rules to protect the integrity of a diagram element.

Stereotypes
Certain stereotypes are predefined (Standard elements) in the UML. others may be user defined.

eg. «user interface» or «controller».

Tagged Values
The explicit definition of a property as a name-value pair. The name is referred to as the tag.
Certain tags are predefined.
The purpose of a tagged value is to assign a feature to a model element in addition to those features to model element in addition to those featurs already defined in the metamodel.

eg. author="Tom", project_phase=2

Constraints
Defines a Semantic condition or restriction. some are predefined.

eg. {up to 50 alpha characters, including spaces and punctuation but no special characters}.

Comments
It attach freeform text to any element in the model.
Comments may be attached to individual attributes, operations, associations, messages, objects, anything in a UML model.

Comments are enclosed in a box with a "folder" top right corner.

Profiles
A profile is a means to customize the use of UML for a specific domain or platform.
Profiles use a combination of extension mechanisms to tailor UML notation.