The container control sends a WM_CONTROL (in Container Controls) message with the CN_QUERYDELTA notification code to its owner to query for more data when a user scrolls to a preset delta value.

param1
     USHORT        id             /*  Container control ID. */
     USHORT        CN_QUERYDELTA  /*  Notification code. */

param2
     PNOTIFYDELTA  pNotifyDelta   /*  Pointer to the NOTIFYDELTA structure. */