Questions: Root Diagram/Namespaces Diagram/Multiplicities Diagram

Forgive me in case this is a recurring discussion, but I need help to clarify few of my doubts. I was going though UML 2.0 Superstructure, Kernel Package. Here I have following questions,

Root Diagram:

Q1. Can you give a practical case of “Element” <-> “Comment” Composition relationship.
Q2. What is the significance of “union” in ‘{union, subsetsXYZ}’

Namespaces Diagram:

Q.1 Can you please explain me the directional association relationship between metaclasses “Namespace” and “NamedElement”?

Multiplicities Diagram

Q.1 Can you please explain me the relationship diagram between “TypedElement” and “Type”.
Q.2 Here the question is, why “TypedElement” is extending from “NamedElement” and why “Type” is extending from “PackageableElement”?

Probably you have noticed by now that I am a beginner.
I need to clear my doubts to get a good understanding of UML.

So … Help Please

  1. Cos Elements can have comments associated with them, if I remembers correctly, Element is defined in the Elements package… then again as a specialization… cos else comment couldn’t be an element… something to do with circlular refenences.

  2. Not a clue, just imagine it’s a joining.

  3. Namespace is a NamedElement

  4. Cos Type is usually a TypedElement (as well as being a namedElement)
    5 Cos TypedElement is a sort’a NamedElement, and Type is a PackageableElement.

Now go and do some studying… or get yourself some nice pills and sweep the streets