Private Declare Function BitBlt& Lib "gdi32" (ByVal hDestDC As Long, _
                                              ByVal X As Long, _
                                              ByVal Y As Long, _
                                              ByVal nWidth As Long, _
                                              ByVal nHeight As Long, _
                                              ByVal hSrcDC As Long, _
                                              ByVal xSrc As Long, _
                                              ByVal ySrc As Long, _
                                              ByVal dwRop As Long)
Private Const SRCAND = &H8800C6
Private Const SRCCOPY = &HCC0020
Private Const SRCPAINT = &HEE0086
其中下画线的数字代表什么含义?