The CU is essentially a 3rd generation D132/D124 Black Box, but they've changed the name. They now call it a Control Unit. It replaces a Black Box in your layout, and offers much more functionality compared to the old BBs. At the present time it does not come in any sets, and must be purchased seperately. It is needed to operate many of the new digital accessories- the pit lane adapter, the driver displays, the position tower- and it adds different driving modes, the ability to hook up a computer for lap timing/counting, and new ways to program the car chips. Any Carrera wireless controller will work with a Control Unit, or an older D132/D124 BB that has Pro-X support built in. The newer BBs (manufactured since 2008) have no Pro-X support, and they will not work with the DIP switch wireless controllers.
If the reseller lists the wireless controllers as Pro-X, or Pro-X compatable, they will be the DIP switch controllers. The part numbers are also different. The Pro-X sets were part #10100. The newer sets, with LED controllers rather than DIP switch controllers, are part #10106. These are the part numbers for the sets that include two controllers and a receiver tower. The LED controller, bought by itself, is part #10107. The DIP switch controller, bought by itself, is part #10103.