It basically is the size of the grains of powder. 1F is cannon powder. 2F is rifle and musket powder. 3f is small bore rifles and pistol powder. 4F is for the pans of primers. There is said to even be a 5f which if for blasting, but I have never seen it.

The reason some people burn 3f in their rifles is, they get a faster burn. Now you would probably never notice the difference looking or listening, but some claim a more complete burn. You also get a slight increase in pressure which is helpful shooting conical bullets.