The agana-airfield i checked has soil, leaves, shoreline, earth - i painted highly visible colored numbers on the closeup-texures to find out.
I assumed the other airfields use the same set of 4, but as it turns out - a lot of airfields have their own individual set of 4 materials.
Of course you can change the layout - that is what the splatmaps are for.
Dont look at a splatmap as a colorful image, see it as 4 separate black&white images.
If you use gimp load your splatmap, go to Menu Colors, Components, Decompose
then color-mode to RGBA and you get 4 separate black&white images (which you can later Recompose back).
Each one of the 4 images represents a material.
The intensity/weight of the material is done gradually from none (black) to fully visible (white)
There is actually 3D information in the materials-normalmap. for example if you have 2 materials one of them stones and the other sand - a high intensity of the stone and a lower intensity of the sand will let the stone peek through the sand in a very convincing way.
Play around with the 4 black&white images a bit and do some test-patterns/gradients - It’ll click, and you’ll get your head around it.
There is another problem, though. The airfields are baked in the clipmaps as well. Clipmaps are for further away rendering, looking down from higher up or in the distance. the clipmaps are not editable (as far as i know). they are pre-rendered by the mapdesigner.
so if you have a wildly differing airfield-layout with very different colors, it will pop in very visibly when it transitions from the clipmap to the altered splatmap-based render.