A host is a server that sends I/O to a volume on a storage array. A host cluster is a group of hosts. You create a host cluster to make it easy to assign the same volumes to multiple hosts.
You define a host separately. It can either be an independent entity or be added to a host cluster. You can assign volumes to an individual host, or a host can be part of a host cluster that shares access to one or more volumes with other hosts in the host cluster.
The host cluster is a logical entity that you create in ThinkSystem System Manager. You must add hosts to the host cluster before you can assign volumes.