The master–detail relationship links two objects in a strong way: children are close to their parent and are strongly related. So this type of relation is usually used to create a detail record.
Think about adding a Case Detail custom object that stores multiple case details, such as multiple products, addresses, or repeated business data (for example, energy readings coming from all contacts' smart home devices) related to the same service request (Case record). Those details have no meaning without the original Case; that's why we choose a master–detail relationship to design this specific model.
When we create a new master–detail relationship, the first thing to select is the parent object.
Note that a master–detail ...
