I'm confused about the meaning of "external entity" when I'm looking for the Actors of the System.
For example, if the Case Study is a Washing Machine does the Actor "Power" that delivers power to the Machine exists? Is the "Power" an external entity?
In UML, definition of Actor is:
specifies a role played by a user or any other system that interacts with the subject . An Actor models a type of role played by an entity that interacts with the subject (e.g., by exchanging signals and data), but which is external to the subject. (OMG Definition) see reference
When we are modeling any Systems, Subject consider as any things. A module, A component, A subsystem, even a use case and etc. So, everything that is External to the subject and interact to subject, can be an actor.
For this reason Actors are in 3 types and should be in external of subject:
Example 1: Assume that we are modeling ATM. So subject is ATM. So a customer (from external the subject) interact with subject. Then Customer can be an Actor.
Example 2: A software system uses Google APIs. So if assume that subject is Google System, other software that interact with Google can be actors.
Example 3: Assume that we are modeling a module in the system. So, subject is that module. Another modules from external (of our module) can interact with our module and can be actors.
Example 4: We are modeling and focusing on a Use Case. So, the subject is this Use Case. Another use cases from the external of our use case interact with our use case (for example Include relationship). So other use cases can be actors of our use case
Example 5: we are modeling Washing Machine and Washing Machine is our Subject. Any customers that start the machine from external the subject can be an actor to our Subject (Washing Machine).
Example 6: we are modeling Washing Machine Inside (like it's Motor Engine). So Motor Engine is our Subject and Power is in external. So Power can be actor for our system (Washing Machine Inside-Motor Engine)
Finally: The meaning of Subject is related to Viewpoints of Observers of the Subject. After we fixed the meaning of Subject in our project, External the Subject can be easily determined.