The purpose of vCenter High Availability is to protect the vCenter Appliance against failures.
The vCenter High Availability cluster uses 3 x VMs in order to protect the appliance, these are:
The Active mode or primary node is the running state. Runs the active instance of vCenter Server.
The Passive node is a clone of the Primary. Recieves updates from the Active node and synchronises the state. It is identical to the Active node, except that all services are stopped.
The Wintness/Quorum node. that does not take over in the case of a failure. It is purely there to break the tie in case of a split brain/network partition.
This product however only protects the vCenter component.
For a more in depth coverage of protecting the vCenter and PSC with HA. I'd recommend watching the following presentation.
vCenter Server and Platform Services Controller 6.5 High Availability , presented by Adam Eckerle and Emad Younis
"In this video we discuss high availability for the Platform Services Controller which includes how the load balancer works for this solution. Then we go into native vCenter Server high availability (VCHA) and how it can be used to protect vCenter Server when embedded and external."
