We all know that the most important characters in the first book are from the Fourth Wing (hence the name of the first book).
However, if you remember, in chapter 17, Page 197, Violet tells us that they received the new patch called “Iron Squad” which is right next to their “Flame Section” one.
Iron Squad refers to the most surviving members since Parapet.
I believe Iron Flame is literally a play on the “Iron Squad” and “Flame Section”.
UPDATE – We now have the Iron Flame verdict and the result of the theory. So, was it proven or disproven? Please note that the verdict will contain spoilers.