In filter update on July 25th, the new shader import/export support was added:
dif.spec.amod.dif.spec
This shader allows to apply overlay texture using additional opacity/transparency texture over the main texture. The main texture (and optionally a bump-map) uses UV#1. The overlay texture (and it's mask-texture) both use UV#2. The overlay texture is placed in DETAIL2 texture slot and it should be "Affected by MASK.R".
The opacity/transparency texture is placed in MASKMAP texture slot (and UV#2 is assigned). Additionally, opacity texture is "Affected by VertexColor.Alpha". Thus, you can additionally control visibility of second/overlay with per-vertex alpha. E.g. paint per-vertex alpha to white will make overlay texture stronger, painting to black will fade out the overlay texture to invisible.
The shader with .iamod overlay texture seems to be using the same technique, but uses inverted alpha (i.a.modulation). In materials browser properties this is set as Affected by: -> Vertices Color, 1-A.
Explained: dif.spec.amod.dif.spec shader
Moderator: Oleg
Re: Explained: dif.spec.amod.dif.spec shader
Do you have an example applicable to trucks?
Re: Explained: dif.spec.amod.dif.spec shader
This is generally for scenery objects, as this shader does not support "paint" extension and can't be painted to truck color.