Before version 2, a cluster node was a machine on the network that was running Heartbeat.
With the introduction of the new ClusterResourceManager, two extra pieces are also mandatory. The ClusterConsensusManager and the ClusterResourceManager itself.
Although it is possible for a node to be fully recognised by Heartbeat without these two extra pieces, it would not be able to participate in ResourceAllocation in the cluster. Thus there are layers of ClusterMembership.
Ping Node, Primary Node