First level soil vs secondary soils

Usage of soils in terrains is detailed here:

As explained here: Overview of the terrain architecture, first level soils are defining the shape of a terrain. So they contain a lot of meaningful macro level informations. Typically:

Secondary soils however, generally don't contain that much extra data. They can feature the same channels as first level soils (both soils are of the same document kind. The difference is only when added to a terrain: who comes first) but this is rarely needed. Typically we have:

Below is an example of a complex terrain - naked - made with several secondary soils. We can see the color variations in the flat area that result of the first soil color covered by secondary soils that'll retain some color influence from the first level soil: