LamaAdd is simply adding two materials together, controlled by individual weights.
If both weights add up to more than 1.0, you might be reflecting more light than is received by the surface. This may prevent the render from converging normally as it breaks energy conservation!
This is the first material to mix
The second material to be mixed.
The mixed value of material 1. You can also use a map here to control the value
The mixed value of material 2. You can also use a map here to control the value