UMLR — Subsets vs. Redefines

Although I have read the sections in Infrastructure that talks about the meaning of association redefinition and subsetting, and I understand it for the most part, I find myself not sure sometimes which one to use when modeling associations that need to be specialized further down an inheritance hierarchy. This happened to me as I was modeling some parts of the new Diagram Definition metammodel.

For some cases, it is obvious what to do like when you have an association with * multipclity that you expect to be populated differently down the hierarchy so you make it "derived union" in anticipation of subclasses subsetting it. However, for an association with 1..1 or 0..1 multipclity that you expect it to be specialized, I am not sure whether to declare it as derived union or as a regular association (in the latter case, I expect it to be redeined).

These features have very powerful yet not much understood semantics by general practitioners, evident by them mostly being used by metamodelers like ourselves, but not by average users of UML (who understand the difference between is-a and has-a relationships for example and use them extensively). It is unfortunate, since these association semantics do have a good mapping to some popular programming languages (like Java as evident by the Eclipse UML2 implementation) and can help modelers intending to generate code from models had they know how to use them properly.

Maybe we need some section in the spec giving a practitioner some guidance in when and how to use these concepts based on different situations that go beyond explaining what they are for?