The SCCM Architecture describes the functionalities and how it can run on different operating systems. Here is a diagram showing the SCCM architecture: 

Following are the key components of SCCM system configuration: 

• CAS (Central Administration Site): A CAS or central site is the primary site of SCCM and is located at the top of the configuration manager HIERARCHY. Additionally, it is referred to as the top-level site in the order. Besides providing a central point of administration, it allows you to MANAGE devices that are distributed throughout a global network. 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.
• Primary site: The site is used to manage devices directly as well as manage bandwidth when your managed devices are distributed across different locations. Almost every client is connected to the primary site. Here, it should be noted that a primary site can only support child secondary sites. Each primary site is capable of supporting up to 250 secondary sites and the hierarchy of more than 100,000 clients.
• Secondary site: Secondary sites should be installed as child sites beneath primary sites. It is used to deliver management capabilities to devices in locations that do not have a reliable connection to a primary site. Despite the fact that a secondary site generally extends the primary site, it is still the primary site that manages all clients. In order to manage secondary sites, primary site servers are used. Each secondary site has its own SQL database server (SQL Express) that helps establish the connection between clients and primary site servers. Secondary site servers support 5,000 client hierarchies, and they also deploy SCCM clients.
• Distribution point: The distribution point helps the client systems obtain the content (files) necessary for installations (Packages, Applications, OS, Driver Packages, etc.). This can be either a local or remote distribution point. Distribution points can support up to 4,000 clients, and by default, primary sites and secondary sites are also distribution points.