0 or - indicates that the keyspace in question is unsharded. This means that a single shard covers the entire keyrange. Note that while both 0 and - are supported for legacy reasons, you can’t merge into shard 0 only -. For this and other reasons, it is recommended to use - instead of 0.

On the other hand a sharded cluster will have multiple keyranges, for example -80 and 80- if you have two shards. Note, that you can still manually target a single shard from your sharded cluster. You can read more about that here.