As per today, shards within are identified by their id attribute, a user-selectable value that must be unique within a cluster: the id of the shard is set upon addShard invocation, with the possibility of assigning the same value of a previously removed shard.

The possibility of having "multiple incarnations" of a shard within a cluster is a potential source of ambiguity for systems managing resources associated to a single incarnation (such as change stream events); the introduction of a shard UUID could help address such a scenario.