Composite map is not supported by most engines, because it basically isn't very efficient. It also only makes sense for something that is composited across the whole geometry, like a lightmap. A decal is usually used only in one spot, but using Composite means Max has to calculate the compositing across the whole mesh to…
composite layers can be used quite a bit but not for decals, they would be to time consuming to setup, floating polys is the most widely spread. use composite for large areas such as floors, plain walls, and cliff faces etc