One of them talks directly to your trains via Bluetooth. This includes more recent models in the LionChief and Legacy lines. This would replace the LionChief remotes. It has all the features you would need to run LionChief engines and Bluetooth equipped Legacy engines can be run with it too but you would be missing out on advanced Legacy features.
The other requires the new Legacy base and is much more feature rich, basically the same as a Legacy remote. I am much less knowledgeable on this app because I don’t own any legacy engines. I’m sure someone with more info on this app can chime in.
They are described in the Lionel catalog pages 8 and 9.