When you buy a new ship, take a look at the modules it comes with by default. You’ll see that your ship has, for example, a size 4 cargo rack in a size 5 optional internal slot. And that’s how they get you … ; )

Class = size
Rating = quality

Ratings:
A – Best all round
B – Better performance than C but higher mass = better integrity for subsystems but will lower your jump range and agility (except with thrusters where agility will be improved over lower ratings)
C – Good all round performance for the price
D – Better performance than E, lowest mass of all other ratings
E – The worst (cheapest) all round performance