What he means is its printed onto a material which allows the driver to see out the rear even when its printed on - due to the tiny holes in it. You will notice that buses have these on when wrapped in graphics, you can still see out when sat on the bus, but only see the graphics from the outside.
Any sign company that does digital print can do this. A cheaper option would be to just print onto block out vinyl but your view from inside is then gone, but its a van anyway, so not really needed.