CAS (Central Administration Site) : The central administration site is the top-level site in a hierarchy. When you configure a hierarchy that has more than one primary site, install a central administration site. If you immediately need two or more primary sites, install the central administration site first.
In CAS, only primary sites are supported as child sites, and a single CAS can support 25 child primary sites so as to extend the scale of your hierarchy. This configuration is necessary because a CAS cannot manage devices directly, which is what primary sites do. CAS is used primarily for reporting and administration purposes.
Ex :
1 CAS = 7 Lakh+ Clients
1 CAS supports how many Primary sites = 25 Primary sites
