A parent project is the base from which a subproject's members, user groups and roles, with their corresponding permissions, are derived. A subproject can inherit project members, user groups and roles from its parent project.
You can create a parent project to track and manage several smaller assignments as subprojects.
When you define users and roles with specific permissions in a project, those users and roles are passed down to any subprojects that belong to that project. This helps you avoid the repeated effort of defining users, user groups and roles across projects.