The two Generacs I installed at work a few years back had an 8 or 10 slot breaker panel built into them. They were 50A switches for 10Kw LPG gensets.
The idea is to feed the switch from your main panel with a 50A breaker and move you essentials to breakers in the xfer switch itself (essentially a subpanel) - fridge, pump, lights, ac, etc. When the power goes out the xfer switch flips from house feed to generator automatically and then back after house power returns. I'd consider this safer than feeding a 200A switch into your 200A panel - safer for the genset that is. A 200A panel is going to require a far larger genset than 10-12kw to carry the load in that panel.