reshardingConfiguration

Specifies the preferred availability zones for each node group in the cluster. If the value of NodeGroupCount is greater than the current number of node groups (shards), you can use this parameter to specify the preferred availability zones of the cluster's shards. If you omit this parameter ElastiCache selects availability zones for you.

You can specify this parameter only if the value of NodeGroupCount is greater than the current number of node groups (shards).