SCCM 2012 changed the way a central site/hierarchy works.
If you think about CM07, any primary site on the top of the hierarchy was automatically a central site and it could manage clients as well.
Also, on a 2007 version, you could have a primary site under a primary site creating a chain of primary sites. On 2012 version, you can have only 1 layer of primary sites and a primary site cannot have another primary site as a child site.
On 2012 version, Microsoft added the CAS (Central Administration Site) and If a primary site is a child site, it will be a child of a cas server and doesn’t have Management Point and cannot perform client actions, ie: client discovery.
There are lots of information on the internet when/when not install a CAS, but basically:
Each Primary site supports up to 250 secondary sites, up to 250 DP/PXE and up to 100k clients machines attached.
Based on this:
When Install a CAS:
1- CM hierarchy will have more than 100k clients in 5 years
2- Political reasons
3- Political reasons (server x desktop) that cannot be accomplished by RBA
4- Need >= 5k Distribution Points
When NOT Install a CAS:
1- Less than 100K (even after 5 years)
2- No political reasons
3- Money issue (Another SQL, System Center, Windows, etc license)
4- Need many clients settings
5- Manage network bandwidth for content (this is done on the DP now and all DP’s can be set as prestage content)
**Each primary site supports a combined total of up to 5,000 distribution points. This total includes all the distribution points at the primary site and all distribution points that belong to the primary site’s child secondary sites.