If work in the planner gets cancelled or put on hold, you can move the work back into the backlog so your plan does not fill up with work.

Open the work details panel and select send to backlog.

Select move to backlog to move the work from the plan into the backlog. 

Copy to backlog will create a copy of the work in the backlog but still leave the work in the plan.

When you move work from the plan into the backlog, Kelloo summarizes the outstanding work and records it against the backlog item along with the resource and role currently allocated.