Well the thread that Jbolt directed you to explained pretty much everything, the XL, SE, SA are just different versions of the Trex, I have XL in my sig cause thats what I fly, and like like Nitrohog said the SE V2 has the alum upgrades where as with my XL I had to buy all the alum parts separate. No one obviously knows the exact definition of the abbreveations and that really shouldnt be of any importance to you, you should be concerned with how much you have to spend and what bird to get.