Right, wrong, wrong, wrong.
Boy, there is more nonsense about this than almost any other topic. This is becase people do not understand the geometry involved.
F marked is for carbine length, NOT "flat top".
Go to this thread and scroll down to the 10th post (by augee) with the picture and you will understand. Look at the pic in below and you will understand.
The line of sight (rear sight to front sight) must converge with a line coming out of the barrel matching the bullet travel. Because the sights are above the barrel, the line of sight must angle down toward the barrel. You can essentially construct a triangle using the barrel, and the sights. If you move closer to the rear sight, the distance between the lines is larger.
The carbine has a shorter sight radius (distance between front and rear sights), and therefore needs either a longer post, or a higher platform for the post in order to zero it. The F marked FSB allows the use of the same post height regardless of sight radius.
This will likely turn into a "pissing match" between the "flat top" and "carbine length" camps, but if you look at the picture you will UNDERSTAND FOR YOURSELF which is correct, and not just repeat errors that you see others propagating.
ETA: Thanks usmc for importing the pic in the next post. I couldn't figure out how to do that.
OOO! I did it, I did it!