Niagara User Param Binding

I’m trying to make Niagara randomly pick a material from the array on a mesh material override. In doing so I came to realize I don’t understand the “User Param Binding” at all. The UE4 documentation is not any help.

What is the User Param Binding supposed to do, and what kind of value does it take?
Also I have not successfully been able to create a random material picker this way, so any help there is appreciated.

You can create User.MaterialInteface variable to bind it:

And then set it in blueprint:


This definitely helps, thank you.

This also revealed a slight glitch in my system that might have been the real cause of my problem. Regardless, this is great, thank you!