Solidfire Cluster nodes are the actual physical nodes (SF series or HSeries nodes ) on which the Element Software is installed.
Ensemble nodes are the node role type assigned to some of the nodes in the cluster. Since Solidfire has distributed filesystem architecture, some of the nodes in the cluster handle Quorum calculation, Configuration management, etc and these nodes are called ensemble nodes. Quorum always maintained by an odd number of nodes hence In a four-node cluster, there will be 3 ensemble nodes and in five-nodes and above clusters, there will be 5 ensemble nodes. This is a cluster defined configuration and users cannot modify or define which nodes can be ensemble nodes.