So, in the example in that document, if you wanted to change the copper portions of the rocket to something with animated flames (because then it will go faster) - you would need to make a new material. If you had a Material Attribute Layers stack that blended together a Rocket_Body layer and a Rocket_Details layer with the mask, you could change Rocket_Details to any Material Layer you wanted (AnimatedFlames, Psychedelic, or Copper) without needing different master materials.
Does that help clarify a bit?