A binder casette is a like a sleeve for binders, you can store binders inside and they will be protected. allow for easy side by side stack or just on top without putting strain on the binder itself.
It’s also really easy to take binders in and out of them.
I’ve posted what they look like here 
!! :
www.elitefourum.com/t/how-to-store-complete-sets/15957/1
Since i’m not near my binders and cant take a pic atm