This feature is available in all editions.
How can you define and keep track of backlog when you don't know which project it will be assigned to?
Introduce a project rollup level into the hierarchy to hold backlog when they are initially defined. Once a project is selected to do the work, the backlog can be moved down the hierarchy into the appropriate project.
Your project hierarchy is a flexible and powerful tool for Release development. By defining a rollup project level to house new backlog as they are defined will allow you to move them into projects or into portfolio items when it makes sense to get working on them.
Moving features or backlog down into a lower level in the project hierarchy is typically done using the Release Planning page, where you can drag and drop backlog defined at one level into any of the direct child level projects in the hierarchy. It's best to be in the Single-Level Project view when doing Release Planning.
Another option for moving backlog around the hierarchy is the "Move" command, available in the "More..." actions list on the Backlog page and other pages. The Move command allows you move multiple levels at once through the hierarchy, or even across the hierarchy in a single step. See How Do I move a backlog item to a different project?