Explanation/Reference:
Ref: http://technet.microsoft.com/en-us/library/gg675084.aspx
CAU is an automated feature that enables you to update clustered servers with little or no loss of availability during the update process. During an Updating Run, CAU transparently performs the following tasks:
* Puts each node of the cluster into node maintenance mode
* Moves the clustered roles off the node
* Installs the updates and any dependent updates
* Performs a restart if necessary
* Brings the node out of maintenance mode
* Restores the clustered roles on the node
* Moves to update the next node
For many clustered roles (formerly called clustered applications and services) in the cluster, the automatic update process triggers a planned failover, and it can cause a transient service interruption for connected clients. However, in the case of continuously available workloads such as Hyper-V with live migration or file server with SMB Transparent Failover, CAU can coordinate cluster updates with no impact to the service availability.
Note
The CAU feature is only compatible with Windows Server 2012 R2 and Windows Server 2012 failover clusters and the clustered roles that are supported on those versions.