When you have a complex object structure (element classes) and need to

Short answer: perform operations on them that vary. For example, in cases where you have a set of classes that are part of a complex hierarchy (like a shopping cart with various types of products), and you need to add new behaviors without changing the objects themselves.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.