Reichu: We first heard about the ATF back in episode #01, although, even here, they don't bother explaining it at all. As the hapless audience, I suppose we're expected to just nod our heads, hang on for the ride, and hope the strange jargon that's thrown our way is somehow elaborated on later.
I should point out that "A.T. Field" is used throughout the series as shorthand for what is technically a defensive/offensive emanation of the A.T. Field. (You'd be surprised how much confusion this creates!) The ATF itself is an invisible force. This really isn't cleared up until #24 -- and, even then, a lot of people still don't get it (understandably).
