The image shows three applications at the top of the diagram pointing to each of the 3 shards at the bottom of the diagram.

Each application has three arrows, each pointing to one of the shards, indicating that the data is being sharded and sent to the 3 replication unit leaders, of which one leader resides on each shard. The two followers of those replication units each reside on the two other shards.

Within the shards, each leader has two arrows pointing to the 2 followers of that replication unit, indicating the replication taking place from each leader to its two followers.