It refers to quality.
A= Best, has all the bells and whistles and made from the best components.
B= Better than C,D, and E but not as good as A made with cheaper components doesn't preform as well
C= Average quality and performance
D= Bad quality
E= Worst quality.
My problem is I am trying to find C3/G or F meaning C Quality 3 Class (large) Gimbaled or Fixed. But I just looked at edshipyard and I can see that Pulse Lasers don't come in C quality for class 3, which is weird because Beam Lasers do I assumed if Beam Lasers come in 3 Pulse would also.