In Aggregator project Top Backlog
When program manager drag'n drop a Backlog Item into a Milestone
Then the mirrored milestone in the Contributor project where the Backlog Item belongs to is updated to add this backlog item
In Contributor Top Backlog
A product owner in a Contributor can neither un-plan nor change the priority of items in the mirrored milestone.
A sub-milestone can be created and the backlog item can be moved in a sub-milestone of a mirrored milestone.
The backlog item can be broken down into sub-backlog items (eg. tasks)