Decals in the context of DECALmachine, are mostly - but not necessarily - single-polygon-meshes with a specific Decal material applied. The material determines the type of a decal, of which there are currently 4:
- Simple - normal mapped + parallax, 1 material component
- Subset - normal mapped + parallax, 2 material components
- Panel - normal mapped, up to 3 material components
- Info - color mapped
3 simple, subset, panel and info decals, each 1 polygon
These so called mesh-decals are an alternative approach to detail texturing.
But unlike actual texturing, detailing using Decals, does not require any UVs being present and is extremely flexible, due to it being noncommittal, non-destructive and object based.
This is not to be confused with mixing textures and materials via multiple uv channels on the same objects, for which term Decal is also sometimes used.
Parallax is a shader effect, that by utilizing an UV offset, creates the illusion of depth on flat geometry.
How well parallax works depends on the angle you are looking at it from.
Also, parallax works better for some types of detail than others.
Going into the surface, works surprisingly well. Going above, not so much.
A single level of depth works better than multiple levels.