Use Case Documentation Skill

Purpose

Describe how a user interactions with a system to achieve a specific goal. Use cases capture functional requirements from a user-centric perspective, often used in Waterfall or Hybrid environments where detailed specification is needed before implementation.

When to Use

• Complex transactional systems (banking, healthcare).

• When specific alternate flows and error handling must be rigorously defined.

• Regulatory compliance requires detailed documentation.

• Bridging gap between business needs and technical functional specs.

Anatomy of a Use Case

Key Elements

• Use Case Name: Verb-Noun phrase (e.g., "Withdraw Cash").

• ID: Unique identifier (e.g., UC-ATM-01).

• Actor(s): Who interacts? (Primary: Initiates; Secondary: External systems).

• Description: Brief summary of goal.

• Preconditions: What must be true BEFORE this starts? (e.g., "User is logged in").

• Trigger: What starts the use case? (e.g., "User clicks Withdraw").

• Main Success Scenario (Basic Flow): The "Happy Path" steps.

• Alternative Flows (Extensions): Variations or error paths.

• Postconditions: What is true AFTER reliable completion? (e.g., "Balance updated").

• Business Rules: Logic links.

Use Case Template

Example: Withdraw Cash at ATM

ID: UC-ATM-01 Name: Withdraw Cash Primary Actor: Bank Customer Secondary Actor: Bank System (Backend)

Description: Customer withdraws physical cash from their checking account via ATM.

Preconditions:

• ATM has sufficient cash.

• ATM is online.

• Customer has valid card and PIN.

Trigger: Customer inserts card.

Main Success Scenario (Happy Path):

• Customer inserts debit card.

• System validates card readability.

• System prompts for PIN.

• Customer enters PIN.

• System validates PIN with Bank System.

• System displays Application Menu.

• Customer selects "Withdraw Cash".

• System prompts for Amount.

• Customer enters Amount (e.g., $100).

• System checks daily limit and account balance.

• System authorizes transaction.

• System dispenses Cash.

• System prints Receipt.

• System ejects Card.

• Use case ends.

Alternative Flows:

A1: Invalid PIN (at Step 5)

• System displays "Invalid PIN" message.

• System prompts to re-enter PIN.

• Customer re-enters PIN.

• Resume at Step 5.

• Constraint: If PIN invalid 3 times, perform A2 (Lock Card).

A2: Lock Card (from A1)

• System displays "Card Locked due to attempts".

• System retains card.

• Use case ends.

A3: Insufficient Funds (at Step 10)

• System determines account balance < requested amount.

• System displays "Insufficient Funds".

• System prompts to enter lower amount or cancel.

• Customer enters new amount.

• Resume at Step 10.

A4: ATM Out of Cash (at Step 10)

• System checks internal cash dispenser > requested amount.

• System detects low cash.

• System displays "Unable to dispense full amount".

• Use case ends.

Postconditions:

• User account debited by withdrawal amount.

• Transaction log created.

• Cash inventory updated.

Use Case vs. User Story

Feature Use Case User Story

Focus System interaction steps User value/goal

Detail High (Flows, Errors) Low (Conversation starter)

Format Document/Structured Text card/Post-it (Who, What, Why)

Lifecycle Update/Maintain over time Done and discarded/archived

Context Waterfall/Hybrid/Complex Agile/Scrum

Best Practices

• Write in Active Voice: "System validates PIN" (not "PIN is validated").

• Keep it System-Neutral UI: Don't say "User clicks blue button at top right". Say "User selects Option".

• Focus on Goal: Don't get lost in technical details (e.g., "System sends JSON packet").

• Link Exceptions: Ensure every error path is handled.

Tools

• Lark Docs / MS Word: Standard formatting.

• Visual Paradigm / UML Tools: For Use Case Diagrams.

• Gherkin: Can translate Use Case flows into Gherkin scenarios for testing.