This page gives information about the Alpha Render Element.
||Render Setup window|| > Render Elements tab > Add button > VRayAlpha
VRayAlpha has no controllable parameters.
The Alpha Render Element is primarily used as a mask to enable other elements to be placed behind the Beauty composite, such as the Background Render Element. The Alpha Render Element shown below takes into account the transparency of the glass in the windows, including the frosted glass made semi-transparent by a texture mask in the glass material.