Returns an aggregation of the bottom n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $bottomN returns all elements in the group.
5.2
https://www.mongodb.com/docs/manual/reference/operator/aggregation/bottomN/#mongodb-group-grp.-bottomN
Returns an aggregation of the bottom n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $bottomN returns all elements in the group.