An application can associate data with the list items in a combo box. The CB_SETITEMDATA message associates a DWORD value with a list item, and the CB_GETITEMDATA retrieves the value associated with a list item.
The example in Creating an Owner-Drawn Combo Box uses item data to associate a constant with each item in a drop-down list box. Such a unique value identifies each item independent of its sorted position.
Other applications might use item data to associate a handle or pointer with a list item. If so, an application can process a WM_DELETEITEM message to delete or free the specified object when the list item is deleted.