Pass Index 2.8 What is it?

There is no Index Node in Composer 2.8. So, what is this?

Isn´t that simply the material ID?

Ok, for what?

Helping in post. Masking, compositing… and alike.

This is what I need

At least supply a timecode. I’m not watching a 1h15m video in the hopes of maybe spotting what you need.