Since not only humans can
be actors, but also other systems, for example, UML allows us to use any other
symbol for an actor. For example, a box is a common symbol for an external system
that is an actor (see also Section 5.1).
An actor is connected with the use cases it participates in by an association.
An actor can additionally be connected directly with the system class it interacts
with. We use this property in the system context diagram.
The solid line is an association. It is the same model element that is used
between classes. This means that we can also state role names and multiplicities.
However, they are generally omitted in the diagram.
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Multiplicity has the following meaning here: At the actor ??™ s side the multiplicity
describes how many concrete individuals or systems participate in their roles as
actors in the use case. The use case open safe in Figure 3.32 requires exactly two
managers. Each of the two managers can deactivate part of the safe ??™ s security system
to open the safe.
At the use case side, the multiplicity describes how often the use case can be
executed concurrently by the actors.
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