UML
extensibility

UML defines three extensibility mechanisms to allow modelers to add
extensions
without having to modify the underlying modeling language. These three
mechanisms
are stereotypes, constraints and tagged values.
Each project comes with a ‘UML extensibility’ node. UML Stereotypes and UML Constraints are
gathered in this node.
UML allows the addition of a pair of string (a tag string and a value
string) on every
model element. User-defined properties provide the same
functionality, however the
value can be of any type (string, integer, boolean, and so on). Thus User-defined
properties do the same as UML tagged values, but they do it better.
See User Properties
for more details.