This page gives information about the Alpha Render Element.
The Alpha Render Element shows the transparency of objects in the scene as grayscale colors. Solid geometry is represented by solid white, while those areas not blocked by geometry within the scene are black. Pixels at antialiased edges have varying shades of gray, as do semi-transparent surfaces.
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 takes into account the transparency of the glass in the windows and masks them. See how the render looks before and after compositing.
When using transparent materials such as glass, make sure that the material is set up to treat the material as transparent with regard to the Alpha channel. For example, when using VRayMtl as the material, set the Affect channels parameter for both Reflection and Refraction to Color+alpha or All channels. Otherwise, the transparent surface is not treated as transparent in the Alpha Render Element.
VRayAlpha render element is available as an output only when VFB is disabled from Render Setup > V-Ray tab > Frame buffer rollout > Enable built-in VFB, and an output format is chosen from Render Setup > Common tab > Render Output > Save file.