The IUnboundHandler interface needs to be implemented by the user, when he wants to use the control in unbound mode. An object that implements IUnboundHandler interface needs  to be passed to UnboundHandler property of the control.

Here's the definition of the  IUnboundHandler interface

[
    uuid(BA3AA5FA-5B09-40F6-80DF-B051C20150B6),
    pointer_default(unique) 
]
interface IUnboundHandler : IUnknown
{
    [propget, id(1), helpcontext(3001), helpstring("Gets the number of items.")] HRESULT ItemsCount( IDispatch* Source, [out, retval ] long* pVal );
    [id(2), helpcontext(3002), helpstring("The source requires an item.")] HRESULT ReadItem( long Index, IDispatch* Source, long ItemHandle );
}