The actor in a use case diagram represents the people and systems that interact with the system.

Outline:

• Hook and context: why actors matter in use case diagrams as a window into a system’s interactions

• What an actor is (and isn’t): the people and systems that interact with the system

• Why both people and systems matter: a holistic view that helps requirements and design

• Types of actors: primary vs. secondary; concrete examples

• How to identify actors in a real project: practical tips

• How actors connect to use cases: the associations and the boundary between system and world

• A clear mini-example: a shopping site that shows actors at work

• Why this thinking helps stakeholders and teams: better communication, fewer surprises

• Quick recap and takeaway

Now, the article:

What’s the role of the actor in a use case diagram? If you’ve spent time with the IREB Foundation Level topics, you’ve probably seen how a use case diagram tries to map out a system’s interactions in a clean, bite-sized picture. The diagram isn’t about the code or the screens alone; it’s about people and other systems that touch the software. So, what does the “actor” actually do in that picture? The correct answer—The people and systems that interact—isn’t just a trivia line. It’s a core idea that keeps requirements grounded in reality.

Let me explain what an actor is. In a use case diagram, an actor represents any entity that interacts with the system under study. That includes real people—think customers, administrators, or support agents—and it also includes other systems that exchange information with your system, like payment gateways, inventory databases, or external services. In short, an actor is a boundary-spanner: someone or something outside the system that can influence or be influenced by what the system does. This isn’t an invitation to pile on every possible party, but it is a reminder to cast a wide enough net to capture meaningful interactions.

Why bother including both people and systems? Because the world doesn’t break neatly into “human users” and “non-human partners.” Many requirements come from how the system talks to other systems, or how those systems push data into or pull data from it. If you pretend only humans matter, you’ll miss important interactions. You might forget that a vendor’s API, a payment processor, or a warehouse database can change behavior in ways that ripple through the system. Including both kinds of actors helps teams see the broader ecosystem in which the software operates. It also helps stakeholders from different domains—business, IT, operations—speak a shared language about what must happen and when.

Let’s talk about actor types. You’ll often hear about primary versus secondary actors. A primary actor is someone or something that benefits directly from the system’s success. For a streaming service, the viewer who watches a show is a primary actor. A secondary actor, on the other hand, might not directly use the system, but it still triggers or supports its functionality. A billing system that handles payments is a classic secondary actor: it doesn’t watch videos, but without it, purchases don’t happen. The distinction isn’t about importance; it’s about the direction of value and the flow of events. And yes, sometimes an actor can be a person, and other times it’s a system, or a combination of both.

How do you identify actors in a real project without chasing every rabbit down the hole? Start by mapping the real-world workflows. Watch how users interact with the system (or how other systems feed data into it). Interview stakeholders from different areas—sales, operations, IT, customer support. Ask concrete questions: “Who triggers this functionality?” “What outside systems does this process rely on?” “Who gets notified when something happens?” A good trick is to prototype a rough list of actors and then test it against a few representative use cases. If someone or something would cause a change in the system’s behavior, that entity probably deserves a spot in the diagram.

Now, how do actors connect to use cases? The core idea is the association—a simple line that links an actor to the use cases they participate in. The system boundary is drawn as a rectangle, and the actors sit outside it, reaching in through associations. This visual cue is powerful: it immediately signals who can initiate an action and who is affected by the outcome. You’ll sometimes see more nuanced relationships, like include and extend, but the essential move is: draw a line from the actor to the use case(s) they interact with. It’s a clean, intuitive way to show responsibilities and touchpoints.

Here’s a compact real-world mini-example to anchor the concept. Picture an online bookstore. You have actors like:

• Customer (a human actor) who browses, adds to cart, and purchases

• Payment Gateway (a system actor) that processes the payment

• Inventory System (a system actor) that updates stock when a sale occurs

• Admin (a human actor) who adds new books and manages orders

Each use case might include things like “Browse catalog,” “Place order,” “Process payment,” and “Update inventory.” The Customer connects to the use cases they trigger (browse, place order), while the Payment Gateway and Inventory System connect to related use cases as external partners that support the flow. The Admin might connect to administrative use cases. Seeing all these connections on one diagram helps everyone understand who or what must be reliable and what kind of information exchanges are needed. And yes, this sometimes reveals gaps you didn’t notice at first—perhaps a missing integration with the shipping service or a need for a new notification workflow.

Why does this matter for requirements thinking and stakeholder conversations? Because actors are the natural starting point for conversation. When you name actors, you’re naming real-world touchpoints. That makes requirements feel less abstract and more actionable. For teams, the actor perspective helps with scope: what must the system be able to do, and with whom must it share information? For project sponsors, it clarifies whose success matters and where risks might lurk—like dependency on a third-party API that’s known to be flaky.

A few practical tips you can use right away

• Start with the boundary: draw the system as a box. Think of actors as the people and systems outside that box that interact with it.

• Name actors precisely. Instead of “user,” specify the role and context: “Shopper,” “WarehouseAPI,” “CRM Integrator.” This reduces ambiguity and helps avoid design detours later.

• Distinguish primary and secondary actors as you go, but don’t let the labels box you in. Use them to spot dependencies and potential failure points.

• Consider external devices and environments too. A mobile app, a point-of-sale terminal, or a smart device can also be actors if they exchange data with your system.

• Don’t treat an actor as a process. An actor is a role, not a workflow. The use cases describe the workflow; the actor represents who’s involved.

• Keep the diagram readable. If it becomes a tangle, split it into smaller views—one for core customer flows, another for back-end integrations, for instance. The goal is clarity, not clutter.

It’s worth noting a tiny tension you’ll sometimes feel. Some teams want to keep diagrams minimal, focusing only on human actors. Others push to show every external system that touches the service. There’s no universal right answer; it depends on your goals and the stakeholders who’ll read the diagram. The trick is to choose a level of detail that supports meaningful discussions today while keeping room for evolution tomorrow.

Connecting this to the broader landscape of foundation-level requirements thinking, actors sit at the crossroads of user needs and system capabilities. They help translate a sea of expectations into a set of observable interactions. When you describe who will interact with the system, you ground requirements in reality. That makes it easier to validate, review, and refine what’s essential for delivering value.

If you’re sketching this out for a team, here’s a quick mnemonic that often helps: A for Actors, C for Cases (use cases), and B for Boundaries. Put actors outside the boundary and use lines to connect to the cases they trigger or participate in. It’s a simple mental model, but it provides a robust scaffold for analysis, discussion, and barter-free negotiation among stakeholders.

A final thought you can carry forward: the actor concept isn’t a one-off checkbox. It’s a living lens on the system’s environment. As projects evolve, new actors may appear, or existing ones may change how they interact with the system. Revisit the diagram regularly, especially when you’re adding new features, integrating with a new partner, or changing regulatory requirements. A fresh glance can spot a missing actor or a misaligned responsibility before it becomes a bigger issue.

In wrapping up, remember the core takeaway: in a use case diagram, the actor represents the people and systems that interact with the system. This inclusive view helps you capture the full range of interactions, align stakeholders, and guide thoughtful design. When you keep actors visible, you keep the map honest—and that makes all the other requirements work more smoothly.

If you want to deepen your understanding, you’ll likely see more examples in your study materials and discussions. But the core idea remains surprisingly sturdy: identify who or what touches the system, connect them to the tasks they influence, and watch how the diagram clarifies pathways, responsibilities, and opportunities for improvement. It’s a small piece of a larger discipline, yes, but it’s a piece that makes the rest of the puzzle fit together with a bit less guesswork and a lot more clarity.