ParentWindow refers to the window handle that underlies the parent control. To designate a non-VCL control as a parent, assign that control's handle to ParentWindow. This assignment causes the control to be moved into the parent's screen area. Setting ParentWindow has no effect if Parent is not nil (Delphi) or NULL (C++). 

TActiveXControl objects use ParentWindow to insert a control in an ActiveX container window. ParentWindow is set automatically when a control is constructed with a call to CreateParented (Delphi) or the appropriate overloaded constructor (C++). 

Use ParentWindow with windowed controls that are packaged in a DLL. This allows references to the control by applications based on other applications. 

CreateParented 

CreateParentedControl 

Create 

Parent 

TActiveXControl