Central administration site with primary sites and secondary sites

This is the most complex scenario we can get. A central administration site may coexist with one or more primary sites--it is the top-level site in the hierarchy. You may consider using central administration if you have two or more very big sites (where the sum of Windows clients, for instance, might be bigger than 150,000), or you would like to separate clients from each site from each other--the legal factor might come into play in this case:

A central administration site does not play any role in managing the clients in terms of actually having some clients assigned to it. You are not able to assign any clients here. It does not process any client data; it just saves data about the whole hierarchy.

With the central administration site and two primary servers connected to it, it is possible--in the case of failure of one of them--to switch endpoints to report to the working one. This feature is the easiest form of high availability provided for endpoints. However, this switch does not happen automatically, and it needs to be triggered from the server console.