The statement has three basic forms to permit specifying a menu or specifying data in words or characters. The data can be in any format, because only your window procedure will use it. The window procedure of the dialog box, window, or control receives this data when the item is created. It is up to the window procedure to process the data.
CTLDATA is often used to supply data that controls the subsequent operation of the custom window. For example, the CTLDATA statement may contain extended style bits - that is, style bits designed specifically for your customized window.
You should reserve the CTLDATA statement for window classes that you create yourself.