Id like to be the next to congratulate you on your win at NYC, and Id also like to say welcome to the Championships!
As you have already gathered, there are 4 different fields within ATL (teams on each field are selected randomly as far as I'm concerned). Each field runs as a normal competition, except for the fact that there are 80+ teams per field; There are qualifying matches, alliances selections, and elimination matches.
Once the winners of each field are found, they all battle it out in semi-finals/finals matches on the 5th field (where FLL champs were held for the 1st day or two).
Competition is exponentially higher than a typical competition, for the most part. Expect matches that will be above 15 points or so. All of the hard-hitting teams within FIRST gather to Atlanta. (217, 67, 111, 148 1114, 25, etc.)
I will guarantee that you will have the time of your life in Atlanta. Each day is jam-packed with excitement. I walked into the Georgia Dome my freshman year and was completely awestruck.
Have fun and Ill see you there! PM me for any other questions or concerns. Id be glad to answer them.