When should you introduce a supertype/subtype structure in conceptual modeling? Choose the most appropriate trigger.

Introduction / Context:
Supertype/subtype modeling is a tool for capturing commonalities and meaningful differences across closely related entity types. The key question is when the extra structure pays off in clarity and correctness.

Given Data / Assumptions:

• There is a general entity with shared attributes.
• Some members have additional attributes or relationships not shared by all.
• The design should avoid large numbers of nullable fields and unclear constraints.

Concept / Approach:
Introduce subtypes when behavior, constraints, or relationships differ materially. A classic signal is a relationship or attribute that applies only to some members of the supertype. Making a subtype isolates these unique properties and improves data integrity and understanding.

Step-by-Step Solution:

Verification / Alternative check:
Compare designs with and without subtypes; without subtypes, unique relationships create nulls or inconsistent foreign keys. With subtypes, the model is clearer and constraints are enforceable.

Why Other Options Are Wrong:

• Same relationships for all subtypes do not justify separating subtypes.
• If attributes apply to all instances, no specialization is needed.
• No attributes applying to any instances is not realistic and does not define meaningful subtypes.

Common Pitfalls:
Overusing subtypes for minor differences; focus on meaningful structural or behavioral distinctions.

Final Answer:
An instance of a subtype participates in a relationship that is unique to that subtype.