In Aggregator Top Backlog

A program manager can remove* a Backlog Item from a Milestone as long as the Backlog Item is not planned in a sub-milestone of the Aggregator Milestone or a sub-milestone of the Contributor project.

There is way to tell the program manager why sth cannot be removed:

• Ideally the item should not be draggable (except for priority changes)
• At very least, if it has to be draggable, on drop there should be a feedback message informing why the drop failed (in which project / sub-milestone the backlog item is planned)

* remove:

• Either move the backlog item back to top backlog
• Or move the backlog item from one milestone to another