Actors represent different types of users. An actor is anyone or anything that exchanges data with the system. An actor
can be a user, external hardware, or another system.
The difference between an actor and an individual system user is that an actor represents a particular class of users,
rather than an actual user.
Several different individual users can play the same role, which means that they can be represented by the same, single
actor. In that case, each individual user constitutes an instance of the actor.
However,
in some situations, only one person plays the role modeled by an actor. For example, there may be only one individual
in the role of system administrator for a rather small system.
The
same user can also act as several actors (that is, the same person can take on different roles).
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