UMLR — Section Unbalanced

The section needs some introductory material. What's here is good, useful and interesting stuff, but its purpose and why it's located at this point in the spec is not sufficiently motivated. The material presented is also somewhat unbalanced. For example, §6.3.1 calls out three model element categories: Classifiers, Events, and Behaviors. Then §6.3.3 contains some nice text on semantics of Behaviors, but nothing further is said about Classifiers or Events. Suggest some text for the two missing categories should be added – if they are important enough to mention, they deserve discussion.
Proposed Resolution: Add described text.