Description:

The 'uniqueKey' field is optional to the $out stage. If not specified and the collection is sharded, the uniqueKey will become the shard key plus the _id (if the _id is not already part of the shard key).

Engineering Ticket Description:

If the aggregation is run in a cluster, the unique key should be the _id and all of the fields of the shard key. Otherwise, the unique key is just the _id.