Zone passes get you use of parks in a given geographical area - and after so many days of free camping, you then pay by the day. Limit 2 weeks per stay, then a week out of the system before you can go to another or same park.
Used to be a VIP level that gave three weeks per park with no time outs, and you could make reservations 90 days out. No daily fees, so "free" camping all year.
The "Elite" and "Premier" levels give you 3 wks per park, no time outs, and 120 day reservations.
There some other classes available on teh resale market with 150 day reservations.
When buying used (which is highly recommended) it's critical that you verify which parks are included in that particular membership. Depending on when and where the membership was originally purchased, it may grant access to less than the full complement of 1000 Trails parks. And some memberships also include some of the non-TT Encore parks.
As fulltimers, we want the most parks we can get. Our current level includes about 90 parks, including all of the 1000 Trails sub-systems. Some are outstanding resorts, some we wouldn't go back to. Most are about average, but getting better now that the economy has improved.
Have fun . . .
al