Just like a class, a stereotype can have properties, which can be referred to as tag definitions. When a stereotype is applied to a model element, the values of the properties may be referred to as tagged values.
The tag definitions are used to define new meta attributes of the extended metaclass, they are used as regular class attributes.
An actual instance of the tag definition is a tagged value (tag).
A tag holds extra information like:
additional information that does not come with UML, for example Precondition for Use Cases.
management data about the state and progress of the project such as author, status, and tested.
language specific data for code generation tools.
A tagged value consists of two parts: name and value (example: Author = Joe).