R5

R5 / 1:Mc

Actor plays zero, one or many Role

Role is played by exactly one Actor

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

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

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