The HA cluster market is split broadly into three types:
The first (Two Node Only) describes any type of cluster whose method of construction does not allow it to expand beyond two nodes. These clusters, once the mainstay of the market, are falling rapidly into disuse. The primary reason seems to be that even if most installations only actually use two nodes for operation, the ability of the cluster to expand beyond that number gives the operator the capacity to add extra nodes at will (whether to perform a rolling upgrade of the cluster hardware, or simply to expand the number of active nodes for greater performance).