The TCM_SETITEMEXTRA message sets the number of bytes per tab reserved for application-defined data in a tab control. You can send this message explicitly or by using the TabCtrl_SetItemExtra macro.
TCM_SETITEMEXTRA wParam = (WPARAM) (int) cb; // number of extra bytes lParam = 0; // not used; must be zero
Returns TRUE if successful or FALSE otherwise.
By default, the number of extra bytes is four. An application that changes the number of extra bytes cannot use the TC_ITEM structure to retrieve and set the application-defined data for a tab. Instead, you must define a new structure that consists of the TC_ITEMHEADER structure followed by application-defined members.
An application should only change the number of extra bytes when a tab control does not contain any tabs.