You're buying a piece of adhesive. That's all.
You have to source your own physical numberplate, so you could buy a flexible one (as shown on their website), or you use your regular one that's on your car now. Obviously a flexible one will contour as shown, a solid one will not. Basically the way to do it would be to either buy another physical plate with whatever you wanted on it then stick that over your existing one, or buy a flexible one and stick that wherever you want to remove at show time.