It’s in the installation instructions. The trick is to understand what this piece is for.
You can’t put a battery operated device inside a wall or inside a light switch box. It creates a fire hazard.
But a lot of people who buy coopers are really fussy about the aesthetics.
So the idea here is that you have an existing single gang regular mains powered wall switch of the same design as the 9500. And you want to put the 9500, which is a battery operated device, on the wall right next to it so that it will appear to be part of a double gang switch, even though the one that’s next to it goes into the wall and this one is going to sit flat on the wall.
So this particular piece (RFBS22) is a dummy plate that goes underneath the other switch (The one that is mains powered) just to lift it enough up off-the-wall that when you then put the double gang face plate over the 9500 and the other switch they’re going to match up exactly in height and give the appearance of a true double gang.
So the purpose of the dummy plate is to lift up the mains powered switch just enough so that it will align vertically with the 9500.
The reason you’re having a problem is you’re trying to use the dummy plate under the 9500. It goes under the other switch.
Then you would put a double face plate like a 9521 over the top of both switches. (Also the bracket underneath is what lets you use the screwless face plate on top; the 9500 is already built for that.)
If you are mounting the 9500 by itself as a single, you don’t need the dummy plate.