SCCM Site hierarchy can divided into:-
a) central site -central location has database with consist of all inventory data for primary site and secondary site.
Central site can link to primary and secondary site
Collection is flow down to primary site and Secondary site.
b) Primary site
Will has database. Suitable when you've IT Administrator to configure policy/deploy package on site.
-Has local control
-Inventory data is push upward to Central site.
Primary site can link to Primary and Secondary site
c) Secondary site
-No database.
-No local control
-Act as proxy in the remote site for client
