a tunnel gets you through thick walls or doors - they can be 2 inches long, or a foot long. Many click together to be as long as you want. The tunnel is open at one end and has the flap at the other end - it is just a way of neatening the sides of the hole you make to install the flap. In a normal door, like mine, you only need a 2" tunnel ...
Make sure the flap is in a room you can seal off. My lot have worked out how to break out so have to leave the kitchen isolated after they are shut in at night cos otherwise the flap ends up in small pieces on the kitchen floor.