What is meant by common control?

In several switching systems, the control subsystem may be an integral part of the switching network itself. This system is termed as direct control switching systems. The systems in that the control subsystem is outside the switching network are termed as common control switching system. Generally Strowger exchanges are direct control systems, while crossbar and electronic exchanges are common control system. Each stored program control systems are common control systems. This common control is also termed as indirect control or register control.