In the item mapper, one can only view items mapped to a collection via the collections they were mapped from.
Just a list of the mapped items would be more useful than the list of collections from where items were mapped.
Furthermore on selection a collection where items were mapped from in order to manage the mapped items the heading is a bit confusing it says:
Browse "Name of the collection the items were mapped from"
Check the box next to items you wish to remove, and choose 'remove'.
This gives the impression one is removing items from the named collection and the context of being in the item mapper is not apparent to the user.