Struggling to find any documentation on some global variables and how to use them.

[Woodpecker]

I need to solve a problem with Arcade that deals with user interaction. Basically, I need to return different objects depending on the selected feature by a user. 

There is a $selection global variable, which I think might actually be what I need, but I am struggling to find any sort of documentation on what it does and how to use it. 

Actually there is quite a few of them that are fairly new to me, but at the same time can't find any resources online. 

Any idea where I could find some meaningful information about those?