UML Collaboration and Structured Classifier

I am still trying to understand the /role Property in the context of StructuredClassifier. If I understand {union} correctly, this means that all elements of the set designated by /role must be elements contained in one of its subsets. Searching the UML reference for the term "subsets role", I find only two subsetting Properties: StructuredClassifier::ownedAttribute and Collaboration::collaborationRole which together would form the derived union of /role.

But why does /role exist at all in StructuredClassifier? AFAICT it would be sufficient to have Collaboration::collaborationRole subset StructuredClassifier::ownedAttribute directly, since Collaboration has no specializations with other possible subsetting Properties to consider. Can a Collaboration have roles, i.e. Properties, which are not also ownedAttributes? Or is /role sometimes used outside of the context of Collaboration?

Yes,collaborationRoles are not necessarily attributes of the Collaboration. Therefore, both sets are subsets of role, but can and often have different contents. collaborationRoles can be attributes of other Classes, parameters of Operations or Behaviors, or variables of Activities (i.e. ConnectableElements).

To add elements to collaborationRoles you probably have to use the specification dialog. At least in Cameo there is no way to do it in the diagram. Any ConnectableElement you find in the model can be used.

For the notation the specification allows to draw a line [...] from the elliptical Collaboration shape to rectangles denoting Classifiers that are the types of Properties of the Collaboration. However, I'm not aware of any tools that support this. Please note that these lines are not model elements. The Activity variable would be excluded from this notation, since it is not a Property.

What most tools do instead is creating an additional - superfluous - association. Of course, this only works for owned attributes. Since this is the most common case, it is not as limiting as it sounds.

For example, figure 11.51 is supposed to show these non-association lines.

The dashed lines in figure 11.53 are role bindings. As the specification says, they are dependencies from the attribute (e.g. broker) to the CollaborationUse. I was able to show them in Cameo, albeit it took a bit of tinkering.

PS: Variables are rarely used and most people have never heard about them.

PPS: There is no notation specified for collaborationRoles that are not Properties. And Cameo can not show attributes not owned by the Collaboration in a composite structure diagram.