What is an overlay texture ? That's a weird option : it allows to superpose a texture on the original texture.

One could display, i.e., continental drift on Earth (or elsewhere), display colours related to elevation, and so on.

The way to display them is the OverlayTexture instruction, a command allowing the texture superposing, only in a AltSurface code (Alternates maps) :

AltSurface "Ocean temperature" "Sol/Earth"
Texture "Earth16k (realistic).dds"
OverlayTexture "earth-overlay-2k-ocean_surface_temp.png"
NormalMap "EarthNormal.dds"
Color [ 0.85 0.85 1.0 ]
SpecularColor [ 0.5 0.5 0.55 ]
SpecularPower 25.0
NightTexture "EarthNight.dds"