R5

R5 / M:Mc-M

Actor participates in zero, one or many Interaction

Interaction involves one or many Actor

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The same Actor can participate in any number of Interactions, each in a potentially different Role.

The UI Actor (domain), for example, could be the initiator of one Interaction and be the target of yet another.

A Role is, by definition, the usage of some Actor in the context of a Interaction.