Well according to the declaration of 'updatelayeredwindow' it's its flags.
From the MSDN help:

dwFlags
This parameter can be one of the following values.
ULW_ALPHA
Use pblend as the blend function. If the display mode is 256 colors or less, the effect of this value is the same as the effect of ULW_OPAQUE.
ULW_COLORKEY
Use crKey as the transparency color.
ULW_OPAQUE
Draw an opaque layered window.
If hdcSrc is NULL, dwFlags should be zero.