Material Override/Render Override?

The gmod lua wiki doesn’t seem to have much info on how Material Override or Render Override work. So what I’m wondering anyhow, is if either of these can target a specific VMT on a model and not override the others.

So say I have a playermodel with a VMT for the eyes, and a VMT for the rest of the body. Would anything work to override the body VMT rather than the eyes’ VMT, or do pretty much all of these just override the whole model’s materials?