The 'is-a' relationship means a subclass should truly be a type of the superclass.
In Java, inheritance should be used when a subclass is a specialized form of the superclass. For example, a 'DigitalCamera' class should inherit from a 'Camera' class because a digital camera is a type of camera, sharing core behaviors but with unique features.
Additional Notes
How does the ‘is-a’ relationship guide inheritance in Java?