Since nobody on here seemed to know the answer, I did some serious amount of posting on many other forums and someone finally got a definitive answer!
(Natl) = Nationwide
(SE) = Southeast
(GS) = Gulf States
Toyota changes their cars a bit for each region. Cars they sell (or are brought into) in CA have to get the special emmissions stuff...
The fact that some models are listed as SR5 SPORT means that they are the SPORT EDITION, which is essentially the SR5 package + extras.
I even called three dealerships and not one sales dude could not tell me the answer to these abbreviations which were listed on their own website! Lame salespeople...