Smartsite iXperion has two separate engines, the Publication Engine to render incoming requests and the CMS Engine, which provides the well-known Smarsite Manager.
No matter the configuration, the fact remains that the Publication and the CMS Engine need to communicate with each other since they are both separate server/application instances running indepedently. Otherwise, the Publication Engine would not be aware of the fact that an item has been added or changed, since that is an action accommodated by the CMS Engine.
On the other hand, since the CMS Engine is basically an updated version of the Smartsite 5.3 runtime engine and it knows nothing about the new SXML syntax, the CMS Engine itself is no longer capable of rendering items. To render an item to e.g. preview your changes in the manager, the CMS Engine needs to communicate with the Publication Engine and let it's render engine handle the preview request.
The basic technology used to communicate between the two server/application instances is Windows Communication Foundation (WCF). This technology offers the possibility to seamlessly switch between named pipes communication (between two separate processes on a single machine), tcp/ip communication (two machines within the same domain) and webservice communication (communication over http), which ever is needed for a specific configuration.