Proper use of Apply Additive?

I can’t get the Apply Additive node to work AT ALL, I was hoping someone here had an example of it in action or could at least explain what it does and how to use it.


Just stumbled upon this question, and I find a very intuitive video about this.

Also, if you are interested, there are 2 legacy documents briefly explained its mechanic. (Just focus on the conceptual part, don’t get into the engine part.)