I've updated and corrected this article based on DAZ's documentation. You should read that if you want more information. This will continue to be more of a "plain talk" breakdown.
Let us begin with the different types of shaders from the "Base Mixing" dropdown at the top. Choosing one of these will hide or gray out some options and reveal others.
The Iray shader behaves as though it has three layers: The "base" layer, the "metallic flakes" layer on top of that, and the "top coat layer" on top of that. It is possible to turn off the top two layers and not use their settings, but the ability to use them is a property this shader has that