In software engineering, what is a use case, and what is the difference between use cases and test cases when describing system behaviour and validation?

Introduction / Context:
Understanding the difference between use cases and test cases is fundamental for both business analysts and testers. Interviewers ask this question to see if candidates can connect requirements modelling with practical test design. A clear view of how use cases drive test cases leads to better coverage and traceability in any software project.

Given Data / Assumptions:
- A use case is part of requirements or analysis documentation.
- A test case is part of the test design and execution artefacts.
- Both deal with system behaviour but at different levels of detail.
- The question is conceptual, with no numerical calculations required.

Concept / Approach:
A use case usually describes a sequence of interactions between an external actor and the system to achieve a goal, such as placing an order or resetting a password. It focuses on business value, actors, preconditions, main flow and alternate flows. Test cases, in contrast, are detailed definitions of how to check that behaviour. A test case identifies preconditions, input data, step by step actions, expected results and postconditions. Often one use case expands into many test cases that cover main flow, alternate paths, boundary conditions and error handling.

Step-by-Step Solution:
Step 1: Recognise that the stem asks for both a definition and a difference. Step 2: Recall that use cases are typically created by business analysts or system analysts early in the life cycle. Step 3: Recall that test cases are created by testers later, often tracing back to use cases and requirements. Step 4: Check each option for a statement that correctly assigns business goal modelling to use cases and verification steps to test cases. Step 5: Option a fits this interpretation exactly, so it is the correct answer.

Verification / Alternative check:
A quick check is to imagine a login feature. The use case would explain how a user logs in successfully and what happens on failure. The test cases derived from it would include specific test data, such as valid credentials, invalid passwords and locked accounts. The relationship is clearly one to many. Option a supports this picture, confirming its correctness.

Why Other Options Are Wrong:
Option b reverses the level of detail and is therefore incorrect. Option c is wrong because using the two terms interchangeably leads to confusion and poor documentation. Option d incorrectly associates use cases and test cases with particular testing types (performance and security), which is not their defining characteristic.

Common Pitfalls:
A common mistake is to write test cases that do not clearly trace back to use cases or requirements, creating gaps in coverage. Another pitfall is creating use cases that contain too much low level technical detail, thereby losing their value as business communication tools. Maintaining a clear separation keeps documentation easier to understand and maintain.

Final Answer:
A use case describes how an actor interacts with the system to achieve a business goal, whereas a test case describes specific input, steps and expected results used to verify that behaviour.