I believe that "high back" would refer to an extra high profile fletching or simply put, "higher than average". 5.5", of course, refers to the length of the fletch which could be very low profile or very high profile equaling more square inches of fletching to catch the wind.
I use custom burned 5.5" "high back" in both shield cut and parabolic on all of the arrows that I make. I like the stability and flight correction they seem to provide. Hunting shots are generally 15 yards or less so drag and speed are a non-issue.