Use Case Modeling: Defining System Boundaries

When you’re building a complex system—whether a medical device, an autonomous vehicle, or a smart home controller—the first challenge is always scope: what’s inside the system, and who or what interacts with it? This is where the SysML use case diagram becomes essential. It’s not just a sketch; it’s a precise boundary-setting tool that defines system functionality from a user’s perspective.

I’ve guided dozens of engineering teams through their first use case diagrams, and the most common mistake? Treating use cases as feature lists rather than purpose-driven interaction contracts. A well-structured SysML use case diagram prevents scope creep, clarifies responsibilities, and aligns stakeholders around shared expectations.

By the end of this chapter, you’ll be able to draw a valid SysML use case diagram that identifies actors, models key behaviors, and integrates with other SysML diagrams. You’ll also understand how to translate functional requirements into actionable use case scenarios—ensuring nothing gets lost in translation between specification and implementation.

Understanding the Role of Use Case Diagrams in SysML

Use case modeling is the cornerstone of behavioral modeling in SysML. It captures how a system interacts with external entities—called actors—to deliver a measurable value. Unlike UML, SysML treats use cases as part of a coherent, traceable model, not isolated diagrams.

At its core, a SysML use case diagram answers three questions: Who interacts with the system? What primary functions does it perform? And how do these functions align with the system’s overall purpose?

Think of it as the system’s “storyboard.” It defines the scope before any architecture or code is written. This isn’t optional—it’s foundational to model-based systems engineering (MBSE).

Key Elements of a SysML Use Case Diagram

Every effective SysML use case diagram includes several well-defined elements:

• System Boundary: A box that encloses all use cases, defining the system’s scope.
• Use Cases: Rounded rectangles representing a specific function or behavior the system performs.
• Actors: Stick figures representing roles played by users or external systems.
• Associations: Lines connecting actors to use cases, showing interaction.
• Generalization: Inheritance relationships between actors or use cases, indicating specialization.

These elements are not arbitrary—they represent real engineering decisions. For example, placing a use case outside the system boundary often signals a need to reconsider system decomposition or external dependencies.

Step-by-Step: Building Your First SysML Use Case Diagram

Let’s walk through a practical example: modeling the user interaction with a smart thermostat.

Step 1: Identify the System and Its Boundary

Start by defining the system. In this case: “Smart Thermostat System.” Draw a box around the name and label it as the system boundary.

This boundary is not a physical limit—it’s a conceptual one. Anything inside is under the system’s control. Anything outside is either an actor or an external system.

Step 2: Identify Actors

Ask: Who or what interacts with the system in a meaningful way?

• Homeowner: Can set temperature, view history, adjust schedules.
• Mobile App: Remote access and control.
• Weather Service API: Provides forecast data to adjust heating patterns.

Notice: The Weather Service API is an actor not because it’s human, but because it interacts with the system. This is a common oversight—many beginners forget that systems can be actors too.

Step 3: Define Use Cases

Use cases describe the functionality from the user’s perspective. Avoid verbs like “manage” or “handle.” Instead, focus on outcomes.

• Set desired temperature
• View current temperature
• Adjust heating schedule
• Receive weather-based suggestions
• Monitor energy usage

Each use case should be a single, meaningful interaction. If a use case includes multiple steps, it’s likely too broad. Break it down.

Step 4: Connect Actors to Use Cases

Draw lines (associations) from actors to the use cases they trigger. For example:

• Homeowner → Set desired temperature
• Mobile App → Set desired temperature
• Homeowner → Adjust heating schedule
• Weather Service API → Receive weather-based suggestions

These associations define the “who does what” relationship. They also become the basis for traceability later.

Step 5: Add Generalizations (Optional)

Use generalization to show specialization. For example:

• Both Homeowner and Mobile App can perform “Set desired temperature” → So, “User” can be a general actor.
• Mobile App may have additional functions like “Send alert” not shared by Homeowner.

Generalization helps reduce redundancy and models inheritance in behavior. It’s especially useful when multiple actors perform similar functions.

Linking Use Cases to Functional Requirements

One of the most powerful features of SysML is traceability. Every use case should be linked to one or more functional requirements.

For example:

• Use Case: Set desired temperature
• Functional Requirement: The system shall allow the user to set a target temperature between 5°C and 30°C.

This link ensures that no requirement is left unimplemented and that verification can later confirm whether the use case was fulfilled.

I’ve seen teams lose critical functionality because they wrote use cases without tracing them to requirements. That’s why in my practice, I always use the functional requirements SysML traceability matrix early in the process.

Best Practices for Functional Requirements and Use Case Alignment

• Write use cases using active voice: “User sets temperature” instead of “Temperature is set.”
• Use consistent naming: “View,” “Adjust,” “Trigger,” “Send” are more precise than “Manage” or “Do.”
• Ensure each use case delivers a clear outcome, not just a process.
• Use generalization only when one actor performs a subset of another’s actions.
• Review the diagram with both technical and non-technical stakeholders to validate scope.

Common Pitfalls and How to Avoid Them

Use case diagrams are deceptively simple. Here are the most frequent errors I’ve observed—and how to fix them.

Mistake	Why It’s a Problem	Solution
Use cases are too broad (e.g., “Manage system”)	Not testable, hard to trace, leads to ambiguity.	Break down into atomic functions: “Set user preferences,” “Generate report.”
Actors are internal system components	Confuses scope; actors represent external entities.	Ask: “Is this entity interacting with the system from outside?” If not, it’s a component.
Missing associations between actors and use cases	Breaks traceability and implies no interaction.	Revisit the scenario: “Who triggers this function?”
Overusing generalization	Creates unnecessary complexity and confusion.	Only generalize when one actor performs a subset of another’s actions.

Remember: The goal isn’t to create a perfect diagram on the first try. It’s to capture a shared understanding. Iterate with your team, and revisit the diagram as requirements evolve.

Integrating Use Case Modeling into a Broader SysML Model

Use case diagrams don’t exist in isolation. They feed into other SysML diagrams like activity diagrams (for workflows), sequence diagrams (for interactions), and requirements models (for traceability).

For example:

• A use case “Set desired temperature” can be detailed in an activity diagram.
• Its interaction sequence can be modeled with a sequence diagram.
• Its functional requirement can be linked in a requirements diagram.

This integration ensures that every use case is not just a diagram entry but a traceable, verifiable piece of the system’s design.

My rule of thumb: If a use case cannot be linked to a requirement and validated through testing, it’s a red flag. The SysML actor scenario must lead to a measurable behavior that can be tested, verified, and documented.

Frequently Asked Questions

What is the difference between a use case and a functional requirement?

A use case describes a user interaction that delivers value. A functional requirement defines a specific behavior the system must exhibit. One use case may satisfy multiple requirements. Use cases are about “what” the system does; requirements are about “how” it must do it.

Can multiple actors share the same use case?

Yes. For example, both a Homeowner and a Mobile App can trigger “Set desired temperature.” This is normal and should be reflected in the diagram with separate associations.

Should I model internal system processes as use cases?

No. Use cases represent interactions with external actors. Internal system processes belong in activity or state machine diagrams. Use cases define the boundary; internal behavior is modeled separately.

How do I know if my use case diagram is complete?

Ask: Does every use case have a clear actor? Does every actor have a meaningful interaction? Can each use case be verified? If yes, and the diagram reflects real system behavior, it’s likely complete.

Can a use case have multiple outcomes?

Yes—but treat each outcome as a separate use case if it leads to different behaviors. For example, “Set temperature” may lead to “Success” or “Error: Invalid input.” The error path should be modeled as a separate scenario, often using an activity diagram.

How do I handle system-to-system interactions in a use case diagram?

Model the other system as an actor. For example, if the thermostat pulls weather data from an API, that API is an actor. This ensures the system boundary remains clear and interactions are traceable.