What parent object can have child objects?

In Workfront, a "Portfolio" is a parent object that can contain multiple programs and projects. Each Portfolio can have a hierarchy of child objects such as programs, which further break down into projects. A project, while it contains tasks and issues, is considered a child within the larger hierarchy of a portfolio. Issues, on the other hand, are typically linked to tasks or projects but are not used as parent objects. This hierarchy enables Workfront users to organize and manage their work across various levels​​​.