What is a use case vs activity diagram?

Question

Accepted Answer

**Use case** — what system does. **Activity diagram** — how it does it.

**Use case:**
- Functionality.
- Actors involved.
- High-level steps.
- System behavior.

**Activity diagram:**
- Process flow.
- Detailed steps.
- Decisions and parallelism.
- Workflow.

**Example:**
```
Use Case: Process Order
Actors: Customer, System
Steps: Submit order, Process payment, Ship

Activity Diagram:
[Start] → (Receive Order) → <Valid?>
                             ↓ Yes    ↓ No
                        (Process)  (Reject)
                             ↓        ↓
                        (Ship)    [End]
                             ↓
                        [End]
```

**Key differences:**
- **Use case** — functionality.
- **Activity** — process flow.

Business Analyst: What is a use case vs activity diagram?