This is the Ambient Rubbish system the VFX team implemented in Mindseye. I made most of the niagara stuff, but my colleague Theo Brunet helped with this as well. We were asked to implement a reactive and dynamic rubbish system to help the world feel more lived in and dynamic. There is a scratch pad which will dynamically set the fixed bounds based upon the spawn box + the buffer size. There's a texture input to determine the kind of distribution of rubbish you want (two rough edges for roadside at the kerb, for example) and the system has several things in place to prevent popping, particles shrinking or growing in front of you etc. You don't tend to see the density of rubbish in Mindseye like I have in the video - but it's easier to demonstrate the things the system is doing with more. And it's more fun to look at.
The other members of the very excellent Barb VFX Team (I'm biased, yes):
Emil Bachvarov
Emmanuel Oujoud
Joel LeGigan (Lead)
Pieter Mans
Theo Brunet
Martin Nielsen (Co-dev, but like one of our own)