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Use Cases - Identify Primary Actors

Working frequently with UML we sometimes wonder if a given actor is a primary actor or secondary. There is never a clear cut answer to that question, because it is always "actor is relative to the system boundary". 

For example, for a eCommerce based sales application, consider a customer as an actor and a customer service representative (CSR) as another actor. When is the customer a primary vs CSR? The answer lies with which system you are considering. If the system in question is primarily geared towards the customer where most actions are achieved for the customer's benefit then the customer is a primary actor. In this case the customer is the primary actor. However a backoffice system which is fed data from the sales system has the CSR as a primary actor vs the customer. 

Please let us know if you have other mechanisms to identify primary vs secondary actors for UML.

Regards, Sandeep