Documentation Request Summary:

Collections now have unique UUIDs assigned upon creation. A sharded or replicated collection has the same UUID on all nodes of a cluster. Similarly, no two collections should ever have the same UUID, even if they are in different clusters.
This means that now there is a difference between restoring a node from a backup (where you want to recreate all collections with their original UUIDs) or restoring an old dump of a collection on a standalone developer machine for example.