It's a whole-panel switch, it works in place of a main breaker with the same function as an interlock kit. Basically like this:
Meter -> T.Panel -> Main Breaker Generator ->
You run the 10/3 or 8/3 line from that to your inlet. This transfer panel allows you to use your old panel without a whole lot of modification without the risk of back feeding the utility line. The generator itself has a circuit breaker of either 30A or 50A which would provide the over current protection if need be. Just make sure that the neutral-ground bonding is removed.