In my school, IT department had 5 professors, who taught different subjects of computer science. School used to manage departments, and professor used to teach. They had their own work life cycles. Everything was so good. But then, school decided to close the department. The professors were bound to loose their jobs. Since they were part of the department. That’s how life is.
When your object is bound to loose it’s existence, when the parent object cease to exist, we say that, child object must have an partOf modifier attached.
For example, An address object is useless, until it is bound to some User object.
Have you heard of cascade-delete in SQL. It describe the same phenomenon.