Get Final Alpha
The input connection is not used by this node (consistent with all Get nodes).
Final Alpha is the value of the surface to be output to the image's Alpha channel (or Alpha element). Alpha is similar to opacity, but for the purpose of compositing the rendered image with other images such as a background plate. The alpha may be different from the opacity, especially where a fully opaque object (opacity of 1) is rendered with an alpha of 0 to allow a background image to show through in a composite.
In Terragen, alphas and opacities have 3 channels to represent potentially different opacities in the Red, Green and Blue channels of an image. This is necessary to correctly composite media with wavelength-dependent scattering or absorption, such as planetary atmospheres and coloured glass.
Although Final Alpha is defined to include atmosphere contributions if they have been applied, in practice Get Final Alpha will always provide the alpha of the surface prior to atmosphere due to the places where you can use it. Future changes to the renderer may expose contexts where functions can be used after atmosphere.