The association role of a link can be <Unspecified>, meaning that it is an unspecified association. This could be useful in design- and even detailed design-phase collaboration diagrams, because you can portray messages that are not passed through relations, such as communication with local or global variables (objects) or communication with variables passed as parameters of a method.
Association roles are themselves not directional, even if they are assigned a directional association. This is in keeping with the emphasis on message traffic, regardless of which class initiated the flow. Once you have created an association role, you can draw the messages that go across it.
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