It's a 1" extension plate that is used if the wall layers are too thick, or if the iBox set too deep and it can't be pulled forward, for whatever reason.

I've used it when the walls are floated mud, a couple of inches thick.

In a typical installation with 1/2" tile backer board and 1/4" tile on the walls, you shouldn't need one.

If you do need one? Shop around. It's usually listed for almost $60 but you can source it for $25-$30.

Thanks, mongo! I think I will skip it buying it. My walls will be replaced with standard Durock and then kerdi. Sounds like I won't need it. If I eventually end up needing it, I am sure the plumber can scare one up.