A control sends a notification message to its parent window to notify the parent about user input or changes to the control. The notification message is a WM_COMMAND message that includes a control identifier and a notification code identifying the nature of the event. A control identifier is a unique number that the application uses to identify the control sending the message. In Windows CE, control identifiers are only valid for child windows.
The application sets the identifier for a control when it creates the control. The application specifies the identifier either in the hMenu parameter of the CreateWindowEx function or in the id member of the dialog box template, which is the DLGITEMTEMPATE structure.
A control must retrieve its identifier before it can send notification messages. A control can use the GetDlgCtrlID function to retrieve its control identifier.