In Exchange 2013 we introduced the concept of “Server Component States”. Server Component State provides granular control over the state of the components that make up an Exchange Server from the point of view of the environment it is running in. This is useful when you want to take an Exchange 2013 Server out of operation partially or completely for a limited time, but still need the Exchange services on the server to be up and running.
One example of such a situation is when “Managed Availability” (MA) comes to the conclusion that a specific server is not healthy in some respects and therefore should be bypassed temporarily until the bottleneck has been identified and removed. MA does so by utilizing so-called “Offline Responders”. They are explained in some detail in Lessons from the Datacenter: Managed Availability. Their counterpart are “Online Responders”, which bring the server back online when it is determined as being healthy again.
Another example is when a server is being updated, for example with a new CU.
Read more at source: http://blogs.technet.com/b/exchange/archive/2013/09/26/server-component-states-in-exchange-2013.aspx