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**A use case** is a description of how a user interacts with a system to achieve a specific goal.
It shows the scenario of using the product through the user's eyes, not the developer's.

### Essence

A use case answers a simple question:
**"What does the user do, and what should the system respond with?"**
It is not about the interface and buttons, but about the logic of interaction - step by step.
For example: "The user resets a password via email" is a separate use case.

### Main elements of a use case

1. **Actor** - the one who interacts with the system.
This can be a person (user, administrator) or another system (API, database).
2. **Goal** - what the actor wants to achieve: register, pay for an order, download a report.
3. **Main Flow** - the sequence of steps when everything goes according to plan.
Example:
  1. the user enters a login and password,
  2. the system checks the data,
  3. the system opens the personal account.
4. **Alternative / Exception Flows** - what happens if something goes wrong.
Example: the user entered a wrong password → the system shows an error.
5. **Preconditions** - what must be true before the scenario starts.
For example, the user must be registered.
6. **Postconditions** - what happens after the scenario completes successfully.
For example, the user is authorized and sees the home page.

### Use case example

**Name:** Password recovery
**Actor:** User
**Precondition:** The user is registered
**Main flow:**

1. The user clicks "Forgot password?"
2. Enters their email
3. The system sends an email with a recovery link
4. The user follows the link and enters a new password
5. The system saves the password and reports success
**Alternative flow:**
- If the email is not found, the system reports an error.

### Why use cases are needed

- They help understand the **system's behavior** before it is implemented.
- They simplify **communication between business and developers** - everyone sees the same scenario.
- They form the basis for **test cases** and **technical requirements**.

### Conclusion

A use case is a tool that describes the **"user ↔ system" interaction** through real scenarios.
It helps all project participants understand equally **what the system does and why**, before coding begins.

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