About Message Handle

[SonicChen]

  TCMDockClient = packed record
    Msg: Cardinal;
    DockSource: TDragDockObject;
    MousePos: TSmallPoint;
    Result: Integer;
  end;
  TMyClass = class
    procedure CMDockClient(Message: TCMDockClient)
      :message CM_DOCKCLIENT;
  end;

here is 3 questions:
1. whether CM_***** message must be registed in window or it's just a number for id?
2. we know window message have 4 param:
sendmessage(hwnd, msg, wparam, lparam)
but when we get message TCMDockClient has a big class
(TDragDockObject) reference
it should have been typecast to Cardinal when sending and typecast back again when receiving automaticly right?
3. why use packed record, what's the benifit?

way to gold shines more than gold