Which tool is used to replicate volumes between PowerFlex clusters?

PowerFlex uses a dedicated replication logic often referred to conceptually as the Replication Engine (technically implemented via the SDR - Storage Data Replicator component in v3, or integrated into the SDS in v4).

Replication Engine (Option C): This component is responsible for intercepting writes to replicated volumes, bundling them, and transmitting them asynchronously (journal-based) to the remote destination cluster. It manages the Journal volumes, handles the bandwidth throttling, and ensures consistency between the source and target sites.

Note: While "Storage Data Server" (B) does the work in v4, "Replication Engine" is the functional answer often used to describe the mechanism of cross-cluster data movement.