“0” or “-” indicates that the keyspace in question is unsharded. Or phrased in a slightly different manner this indicates that a single shard covers the entire keyrange. Note, the reason both “0” and “-” are used is because you can’t merge into shard “0” only “-”.
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.