Pelco Developer Network (PDN)

When exporting from Pelco Aggregation System, why can the PelcoSDK::System schema type NOT be pelcoaggregationdirect?

The schema type pelcoaggregationdirect only returns a list containing one device of type CONTROLLER. This device cannot be used as a camera or system or other type of device.

http://pdn.pelco.com/content/exporting-pelco-aggregation-system