VBuf parameter points to an array of CHAR_INFO structures that contain characters and their attributes to be drawn in the dialog.

If VBuf is NULL, the plugin itself must draw the control using the Text service function when a DN_DRAWDLGITEM event comes.

If VBuf is not NULL, the plugin must fill the VBuf array when it receives the DN_DRAWDLGITEM event, and the dialog manager will then copy the contents of the buffer to the screen.

This is the typical scenario for using the DI_USERCONTROL (see the source code of Reversi plugin):

// allocate memory for the virtual buffer before calling DialogEx
#define DIM(Item) (((Item).X2-(Item).X1+1)*((Item).Y2-(Item).Y1+1))
CHAR_INFO *VBuf=new CHAR_INFO[DIM(DialogItems[11])];
DialogItems[11].VBuf=VBuf;
  .
  .
  .
// in the dialog callback function
struct FarDialogItem DialogItem;

case DN_DRAWDLGITEM:
  if(Param1 == 11)
  {
    char Face[4]="   ";
    SMALL_RECT Rect;
    BYTE AddColor=0x00;

    // get coordinates of the dialog and description of the item
    Info.SendDlgMessage(hDlg,DM_GETDLGRECT,0,(LONG_PTR)&Rect);
    Info.SendDlgMessage(hDlg,DM_GETDLGITEM,11,(LONG_PTR)&DialogItem);

    // drawing the game field
    for(Y=0; Y < 8; ++Y)
    {
      for(X=0; X < 8; ++X)
      {
        // prepare one rectangle

        // if the cell is not used...
        if (GAME[0].Field[Y*8+X]==0)
          Face[1]=' ';
        // for the white player
        else if (GAME[0].Field[Y*8+X]==GAME[0].Pl1)
        {
          Face[1]=FaceWhite;
          AddColor=0x00;
        }
        // for the black player
        else if (GAME[0].Field[Y*8+X]==GAME[0].Pl2)
        {
          Face[1]=FaceBlack;
          AddColor=0x0F;
        }

        // if the memory couldn't be allocated,
        // draw with the Text function
        if(!DialogItem.VBuf)
        {
          Info.Text(Rect.Left+DialogItem.X1+X*3,
                   Rect.Top+DialogItem.Y1+Y,
                  ColorsPanel[Y&1][X&1]|AddColor,
                  Face);
        }
        else // if the memory was allocated, use virtual buffer
        {
          CHAR_INFO *VBuf=&DialogItem.VBuf[Y*8*3+X*3];

          VBuf[0].Char.AsciiChar=Face[0];
          VBuf[1].Char.AsciiChar=Face[1];
          VBuf[2].Char.AsciiChar=Face[2];

          VBuf[0].Attributes=
             VBuf[1].Attributes=
             VBuf[2].Attributes=ColorsPanel[Y&1][X&1]|AddColor;
        }
      }
    }
  }
  return TRUE;

1. For a description of how to manage the text cursor in a user-defined dialog control, see DM_GETCURSORPOS or DM_SETCURSORPOS.
2. The DI_USERCONTROL item is not supported by the Dialog function.