An optional capability on forwarders that helps prevent loss of data when sending data to an indexer. With indexer acknowledgment, the forwarder resends any data not acknowledged as "received" by the indexer.
When forwarding data to a indexer cluster node, indexer acknowledgement helps ensure end-to-end data fidelity.
For more information
In Forwarding Data:
In Managing Indexers and Clusters of Indexers: