Subitems: records inside records — Subitems vs. a plain relation

Both connect two tables, but they mean different things. A plain Relation points at a record that exists on its own and gets reused — a customer, a part in your catalogue, a venue. You pick an existing one. Subitems mean ownership: the child belongs to exactly one parent and is created inside it. You add them, you don't pick them. Delete the parent and they go too. If you find yourself creating a record in one table just so you can link it from another, that relationship almost certainly wants to be subitems. Nothing is copied and no field is added — the child records still live in their own table. Only where you read them changes.