Generally speaking, the "F" (or a little Wing symbol) stands for Flying. Check the official cards for Iron Man and Doctor Doom - there's the official rules text. The idea is that so many characters can fly, well, it's a waste of space to write it out each time.
Otherwise, it differs from creator to creator. You'd have to go to each person's individual thread in the Comic Hero Custom Creations forum... or look through
THIS thread to see the creator.