Hi Zapatista,
The other masks/outputs of the snow macro are images which contain information about what the snow macro device did to your terrain.
For example, if you wish to have thick snow to be more shiny like compressed snow/icy snow, then you can use the "snow depth" mask to mask a reflective shader. The snow depth mask is white where snow is thick and greyish to black where snow is thin or absent.
The snow depth is incorporated into the Terragen output file. Why?
Because the snow macro device simulates snow fall on your terrain, adding height to your terrain by filling depressions with snow and smoothing parts of your terrain.
Masks which define where snow has fallen are not incorporated into the Terragen output. These are just images, like the snow depth mask, which give information on where snow has fallen. White is where most snow has been built up and black is where no snow has been built up.
To see the difference between your terrain with and without snow you can also connect a Terragen output device before the snow macro device.
It's ok to have multiple outputs at different stages in the node network.
Select your output before snow, hit 'f' on your keyboard and press the yellow terrain build button. (yellow builds terrain to selected node)
Hit 'F8' on your keyboard to view the terrain in 3D.
Select your output after snow, hit 'f' on your keyboard and press the yellow or green terrain build button. (does not matter which since it is the last node)
Hit 'F8' on your keyboard to view the terrain in 3D.