I know it can be frustrating to figure out, but will become clearer as you start to use the various systems.
With your TT membership you get 20% off the normal rate at any Encore Park anytime. You need to make that reservation thru the Encore website/ reservations phone #. (Your included in your contract named Encore parks reservations are made thru the TT reservations system.)
RPI reservations need to be made thru the RPI reservations system.
I've also stayed at an Encore park with a Passport America discount on a reservation made with a phone call to the individual park.
My understanding of the 5 year deal is that Encore reserves the right to take a park out of the TT system after 5 years. That it is not an individual contract restriction. Might be wrong about that.