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.
* Primary site can support up to 250 secondary sites (It’s completely depends on WAN speed)
In SCCM 2007 :
Not much difference but in SCCM 2007 Primary can have multiple primary sites.
