In addition to supporting properties and methods, ActiveX objects can be a source of events. In Automation, an event is a method that is called by an ActiveX object, rather than implemented by the object. For example, an object might include an event method named Button that retrieves clicks of the mouse button. Instead of being implemented by the object, the Button method returns an object that is a source of events.
In Automation, you use the source attribute to identify a member that is a source of events.
Details of the Automation event interfaces are provided in Programming with MFC, provided with Visual C++ version 4.1 or later product documentation.