Clustering lets you group multiple Firepower Threat Defense (FTD) units together as a single logical device. Clustering is only supported for the FTD device on the Firepower 9300 and the Firepower 4100 series. A cluster provides all the convenience of a single device (management, integration into a network) while achieving the increased throughput and redundancy of multiple devices. https://www.cisco.com/c/en/us/td/docs/security/firepower/fxos/clustering/asa-cluster-solution.html