A note is a graphical symbol for rendering constraints or comments attached to an element or a collection of elements. Graphically, a note is rendered as a rectangle with a dog-eared corner, together with a textual or graphical comment.
A stereotype is an extension of the vocabulary of the UML, allowing you to create new kinds of building blocks similar to existing ones but specific to your problem. Graphically, a stereotype is rendered as a name enclosed by guillemets (French quotation marks of the form « »), placed above the name of another element.
Optionally the stereotyped element may be rendered by using a new icon associated with that stereotype.
A tagged value is a property of a stereotype, allowing you to create new information in an element bearing that stereotype. Graphically, a tagged value is rendered as a string of the form name = value within a note attached to the object.
A constraint is a textual specification of the semantics of a UML element, allowing you to add new rules or to modify existing ones. Graphically, a constraint is rendered as a string enclosed by brackets and placed near the associated element or connected to that element or elements by dependency relationships. As an alternative, you can render a constraint in a note.
Notes
A note that renders a comment has no semantic impact, meaning that its contents do not alter the meaning of the model to which it is attached. This is why notes are used to specify things like requirements, observations, reviews, and explanations, in addition to rendering constraints.
A note may contain any combination of text or graphics. If your implementation allows it, you can put a live URL inside a note, or even link to or embed another document. In this way, you can use the UML to organize all the artifacts you might generate or use during development, as Figure 6-3 illustrates.