①BUTTON_IMAGELIST
Contains information about an image list that is used with a button control
②BUTTON_SPLITINFO
Contains information that defines a split button (BS_SPLITBUTTON and BS_DEFSPLITBUTTON styles)
③NMBCDROPDOWN
Contains information about a BCN_DROPDOWN notification
④NMBCHOTITEM
Contains information about the movement of the mouse over a button control
【Windows API】Button Control Structures,布布扣,bubuko.com
【Windows API】Button Control Structures
原文：http://www.cnblogs.com/Cxx-Code/p/3632083.html